Darwin's First Botanizing Steps Followed His Geological Ones

 “I collected every plant, which I could see in flower, & as it was the flowering season I hope my collection may be of some interest to you." - Charles Darwin in a letter to his friend and mentor John Stevens Henslow, 1836.

Charles Robert Darwin's interest in the natural world was widespread. As a student, he loved to hunt animals and collected bugs and minerals. His mentor and friend John Stevens Henslow, mineralogist and professor of botany, introduced the young Darwin to both disciplines. Darwin attended Henslow's botany lectures and field trips each year during his three years at Cambridge, visiting also private meetings at Henslow's home. Here he met with Adam Sedgwick, president of the newly formed Geological Society of London. During a geological field trip in the summer of 1831 with Sedgwick, Darwin collected and preserved also some plant specimens.

Herbarium sheet by J. S. Henslow with three plants collected by Charles Darwin in 1831 at Barmouth, North Wales. This is the earliest-known herbarium specimen collected by Darwin.

During the five-year-long voyage of the Beagle Darwin collected plants or seeds on the Cape Verde Islands, in Argentina, in Uruguay, in Chile, in Brazil and some of the visited islands, like the Falkland, Galápagos and Cocos islands. As Darwin had limited space on the Beagle, most occupied by rocks and animals, he limited himself to remote or poorly studied localities.

Darwin had prepared several thousand labels in different colors before the voyage to be applied to every dried plant (the labels including species, locality, date and his signature). Wet specimens, conserved in "spirits of wine", were tagged with a small, metallic plate. Henslow, who back in England managed Darwin's collection, however, removed most labels when including Darwin's specimens into the herbarium. Unlike the collected rocks and animals Darwin didn't number the plant specimens, so it seems a bit confusion sneaked into the collection. Another friend of Darwin, botanist Joseph Dalton Hooker, lamented to Darwin that not all notes could be attributed to the preserved plants.

Darwin's plant collection is especially interesting as it includes many species from less visited islands of the Galápagos and the Cocos archipelago. Darwin was intrigued about the relationship of the isolated species found on the islands to the species found on nearby continents. Later Darwin conducted experiments with seeds, showing that some can survive salty water for months and so be dispersed by marine currents. Despite Darwin's plans, he didn't publish the collected plants in “The Voyage of the Beagle” (published in 1839), as a very busy Henslow didn't meet the deadlines for publication.

Darwin collected 756 different species, subspecies or varieties of vascular plants during his five years long voyage around the world, 220 species were new to science. Darwin was especially surprised by the variability displayed by plants. A collected grass species was divided by Henslow into fifteen different varieties! This was an intriguing observation, important for his later formulated theory of evolution, how one species can split over time in various new ones. Also, the relationship of plant species on islands to nearby continents was an important observation. The plants from the Galápagos islands showed, according to Hooker, a remarkable variability between the single islands, however some even more remarkable similarities to species from North America and Brazil. Would a divine creator not be able to create distinct, unique species on remote islands as he pleased? However, if seeds can be dispersed with marine currents and islands be colonized by plants from nearby continents, couldn't they also evolve there in new species?

An advice for the prospecting geologist from 1731 - observe the water

“The miner needs in his art to have the most experience, so that he knows the place, the mountain or the hill, the valley or the field, that can be mined with success, and to avoid to dig were nothing can be gained.”
from Agricola, "Zwölf Bücher vom Berg- und Hüttenwesen", I. Buch (1549)

Georg Grandtegger was a mine inspector in the Prettauer mine (Tyrol) who published in 1731 a field guide to find ore. Some of his suggestions may be useful even today, so he writes:
 

"The water of a spring must be tasted for the dissolved substances in it"
 

It is true that minerals like salt, sulfur and some metallic ores are water-soluble and can alter the taste and smell of water. Water saturated with metals can also precipitate new minerals (mostly oxides and hydroxides) in a river, like reddish-brown sediments when saturated with iron or greenish crusts when saturated with copper.
 

So Grandtegger continues:

"The brand [a term referring to color-alteration of the rock] comes from an ore along the creek. Follow it as long as you see it, then you will find the ore.”
"If you find in fountains [read springs], feed by the mountain, many reddish, bluish or black stones, or even colored green, even if the rocks itself have no ore, so flows the water out from veins of ore."
 

Fig.1. A spring, the mud around is colored by iron-oxides and -hydroxides, a clue that in the underground there is ore-rich schist to be found.

Grandtegger correctly suggest that a prospecting geologist should observe carefully if rocks are colored by precipitations of metals in a river. If so the geologist can follow the river until the spring. The source of the dissolved metals will likely to be found here in the underground. 

Fig.2. The reddish colors of the pebbles in this creek suggest the presence of iron and copper. Sometimes even the name of certain localities can help the prospecting geologist, like here, as this small creek is found in the “red valley”.

A last important observation, as dissolved copper is poisonous for animals and plants, rivers flowing in copper-rich rocks will likely show a diminished presence of insects and fish – so it may be a good idea to ask local fishermen about spots were they don´t get to catch anything, it may be the right spot for the geologist.

Darwin´s first botanizing steps followed the geological ones

“I collected every plant, which I could see in flower, & as it was the flowering season I hope my collection may be of some interest to you."
Darwin in a letter to Henslow, 1836
 

Darwin´s interests in the natural world were widespread. He enjoyed hunting, later also taxidermy. With his cousin William Darwin Fox he hunted for bugs. He collected rocks and minerals and later geologized around the world during the voyage of the Beagle (1831-1836). His mentor and friend John Stevens Henslow was professor of Mineralogy and later for Botany, introducing the student Darwin in both disciplines. Darwin attended Henslow´s botany lectures, labs and field trips each year during his three years at Cambridge, visiting also private science meetings at Henslow´s home. During the geological field trip in summer of 1831 with Adam Sedgwick he also collected and preserved some plant specimens.

Fig.1. Herbarium sheet by J. S. Henslow with three plants collected by Charles Darwin in 1831 at Barmouth, North Wales. This is the earliest-known herbarium specimen collected by Darwin (image source).
 

During the voyage of the Beagle Darwin collected plants or seeds on the Cape Verde Islands (the first stop of the Beagle), then Argentina, Uruguay, Chile, Brazil, later also on some of the visited islands, like the Falkland, Galápagos and Cocos islands. As Darwin had limited space on the Beagle, most occupied by rocks and animals, he concentrated on remote or less well studied localities.
Darwin had prepared several thousand labels in different colors before the voyage to be applied to every dried plant (the labels including plant name, locality, date and his signature). Wet specimens, conserved in "spirits of wine", were tagged with a metal tag. Henslow, who back in England managed Darwin´s collection, removed however most labels when putting Darwin´s specimens into the herbarium. Unlike the collected rocks and animals Darwin didn´t number the plants, so it seems a bit confusion sneaked into the collection. Another friend of Darwin, botanist Joseph Dalton Hooker, lamented to Darwin that not all notes could be attributed to the preserved plant specimens.
 

Darwin´s plant collection is especially interesting as it includes many species from the – at the time – less visited islands of Galápagos and Cocos islands. Darwin was intrigued about the relationship of the island species to nearby continents, he will do some experiments with seeds showing that some can survive salt water for months and so be dispersed over the sea. Despite Darwin´s first plans he didn't publish the collected plants in his description of “The Voyage of the Beagle” (published in 1839), as a very busy Henslow didn't meet the deadlines for publication.
 
Darwin collected 756 different species, subspecies or varieties of vascular plants during his voyage around the world, 220 species were new to science. Darwin was especially surprised by the variability of plants, one collected grass species was divided by Henslow into 15 different groups! This was an intriguing observation, important for his later theory of evolution, as variability is where natural selection acts on. Also the relationship of plant species on islands to nearby continents was an important observation. The plants from the Galápagos islands showed, according to Hooker, a remarkable variability between the single islands, however some even more remarkable similarities to species from North America and Brazil. Would a divine creator not be able to create distinct, unique species on remote islands as he pleased? However if seeds could disperse with marine currents and islands be colonized by plants from nearby continents, couldn't they also evolve there in new species?
 

References:
 

PORTER, D.M. (2010): Darwin: the Botanist on the Beagle. Proceedings of the California Academy of Sciences, Vol.61(4): 117-156

Geological Prospecting Following The Tales Of Haunted Mines

Myths were already used to reconstruct the geological risk of certain areas and may also be of interest for prospecting geologists. Still many modern localities bear names associated to past mining operations, precious metals or ore. A lateral valley of the South Tyrolean Ahrntal is known as Röttal, “Röt" meaning red and named after the reddish rocks found there. These rocks are ore-rich greenschists, the reddish colors caused by alteration and weathering over time of iron- and sulfur-minerals (often associated with more valuable minerals). Probably this and other geological clues (like rivers poisoned by traces of copper and poor plant growth) helped once to discover the copper deposits deep within the mountain. 

 

Fig.1. View of a small creek in the "red" valley.

According to a local legend the nearby mine of Prettau was discovered when a wild bull throw some large rocks into the air. The owner of the animal noted some shiny minerals inside the rocks and even if not gold, so he had found a rich deposit of copper- and iron sulfides. Maybe this legend reflects the idea of using such well visible geological clues, like minerals or alteration products, do discover the hidden treasures of a mountain.

Mining for metals in the Alps dates back at least for 4.800 years (a 25m long gallery in North Tyrol was dated to 2.800 B.C.), in South Tyrol slag remains were dated to 1.200-1.000 BC for sure. Slag remains found in Ahrntal possibly date back to the early and middle bronze age (3.300-1.800 BC), even if the provenance of the used copper ore is unknown. The extraction of copper ore in the Ahrntal became important only in medieval times, especially in the 15th century. 

Fig.2. Medieval prospecting pit in ore-bearing greenschists (prasinitic) rocks.

So it´s interesting to note that some galleries found in the Ahrntal are, according to local folklore, associated to the Roman dominion. The galleries excavated in gneiss are not especially deep, the longest recorded is just 40m. It´s for sure only superficial prospecting, soon abandoned.

Fig.3. A supposedly haunted pit, entrance to a short gallery excavated into the weathered grey gneiss, in yellow alteration rim.

In local folklore the galleries are called “antrischen Löcher”, "antrisch" an old term to describe something spooky or haunted and "Löcher" simply meaning hole. The antrischen Löcher were inhabited, so the legend tells, by descendants of the first man and women. However as Adam and Eve tried to hide their illegitimate children before god, they now are damned to live in the underground. They are the guardians of underground treasures and eventually will donate the hidden treasures to good people, if they deserve such gifts.

According to historic archives some galleries date back for sure to the year 1530, when a mister Franz Widmair requested permission to prospect for ore in this area. Mines dating back to Roman times are a possibility, even if highly unlikely, as there exists no written record or artifact made of the extracted ore to prove Roman mining operations.

Folklore also tells of silver-veins, even if the petrological composition of the rocks would suggest copper. The found ore is anyway of no economic value nowadays.

Now even if geology contradicts some speculations based solely on local tales (like the galleries dating back 2.000 years and the search for silver), it´s nevertheless interesting to note that without the legends surrounding these artificial galleries and pits these would have probably soon be forgotten. By following and evaluating tales provided by locals a geologist may discover some interesting additional information to include in a geological map, be it abandoned mines, quarries or minerals- and ore-associations. 

The true treasure of the North

GOLD! GOLD! GOLD! found in the Klondike river in the Yukon territory, Alaska. The news spread like wildfire, fueling the last great gold-rush of the United States in 1896-99.
 

Fig.1." The Prisoner of White Agony Creek" (2008).

Also French businessman Loicq de Lobel decided in 1898 to try his luck in the new world. Even if not directly interested in searching for gold, he hoped to make a living by selling equipment to the prospectors. So the family de Lobel, his wife and four children, following the famous Chilkoot Trail ventured into the northern wilderness, first by feet and later by boat. To distract herself from the perils of the voyage, de Loicq´s wife, which name is not recorded, botanized along the way. She collected for the very first time specimens of the endemic lady's-slipper orchid, Astralagus, bearberry, Epilobium, arnica and a blue-flowering bellflower.

 “… everywhere there were nice flowers, at our arrival at Glenora we found lots of flowering plants...”
 

The de Lobel family lived for a time in the Yukon territory, then moved to the Aleuten Islands, to finally return to France. The Klondike Gold Rush ended as suddenly as it began, only few found great riches and fortune. However the collected plants by the de Lobel became known as “Klondike River Herbarium” and represents still today a unique botanic treasure.

Bibliography:

THINARD, F. (2013): Das Herbarium der Entdecker - Humboldt, Darwin & Co. - botanische Forscher und ihre Reisen. Haupt-Verlag: 168

Tiny Plants Creating Big Rocks

Often enough the rocks determinate the presence and distribution of plants (as shown in the wonderful blog "In the Company of Plants and Rocks"), but sometimes it's the plant shaping the rocks. 

Plate showing the deposition of travertine* around single algae cells (ca. 1935). The high content of carbonic acid (white circles) dissolves carbonate (shown as schematic rhombohedra-crystals). Plants (like this alga) use the carbon dioxide for their metabolism and the water becomes less acid, the carbonate is deposited around the plant tissue. The final figure shows the soft water, with less dissolved carbonate.

The upper caption reads: 

"Tiny Plants
built the Travertine of Polling**
Substance in the water, they found
Sun gave them power"

This plate was drawn by the German Prof. Dr. Gustav Dunzinger (1868-1940), pharmacist and plant-physiologist. Dunzinger dedicated himself also to scientific-botanical illustrations.

Fig.2. Outcrop with travertine investigated by Dunzinger, old quarry near the German village of **Polling.

*Travertine is the general term in Germany for continental limestone, however in English it is referred to limestone from hot springs or deposited by inorganic processes. Calcareous tufa forms by precipitation of calcium carbonate from “cool” springs and river waters, helped by organic processes - the travertine of Polling is therefore a tufa.

How the Study of Plants revealed the Variability of Climate

The news of the resuscitated "Ice Age plant", regenerated from 30.000 year old tissue conserved in permafrost, is an intriguing discovery that will help to better understand the evolution of ecosystems during one of the most dramatic epochs of earth history - the Pleistocene-Holocene transition some 10.000 years ago, characterized by strong climatic oscillations from glacial to interglacial conditions.

Already the study of fossil plants played an important role in the reconstruction of the environment and the climate of this period.  The German explorer Alexander von Humboldt (1769-1859) was one of the first naturalists to scientifically discuss the distribution of plants related to environmental factors like temperature, rainfall, latitude, height and soil - one of the most famous results of this work is the depiction of the vegetation belts of the Andes.

Fig.1. Vegetation zonation in the Andes, from the "Berghaus-Atlas", a supplement to Humboldt´s work "Kosmos", published in 1645-1862.

Based on these observations it became clear that the fossil remains of plants can be used to reconstruct the environmental factors during the lifetime of these plants.
In 1876 the Norwegian botanist Axel Gudbrand Blytt (1843-1898) published a book on his observations about the distribution of the Ice Age flora on the Scandinavian Peninsula. He recognized various plant communities - defined as Arctic, Subarctic, Boreal, Atlantic, Subboreal and Subatlantic - and suggested, based also on layers of peat where he observed the same plant communities in a certain order, that these communities were the results of various climate driven migrations waves on the peninsula. For Blytt especially the change of dry and wet periods controlled the distribution and migration of plants. These supposed changes could also explain the discoveries of layers with tree stumps in peats all over Europe, grown there when the climate was more favourable for trees. 

Fig.2. Blytt´s map of the distribution of plant communities in Norway, from BLYTT 1876.

The Swedish geologist Lennart von Post (1884-1951) used particular plant remains to infer past climatic oscillations. Flowering plants produce pollen grains covered by a chemically very stable substance named Sporopollenin, therefore pollen grains usually are well preserved in soils and sediments. The structures, like spikes or pores, on the surface of pollen grains are species-specific and can be used to determine from which plant-species the pollen was produced. Von Post counted and identified many hundred of pollen grains found in specific depth-intervals of sediment cores and plotted the relative percentage of every species in a diagram. He found that there was a succession of different plants; cold periods during an Ice Age were dominated by pollen from trees adapted to cold and humid conditions, like birch or pine. During warmer periods the pollen of these species disappeared and new tree species, like oak and fir, appeared in the pollen diagram. When the landscape finally became occupied by humans the amount of tree pollen decreases, as the forest is replaced with fields of grass or crop and the pollen of these plants dominate in the sediment core. 

The study of pollen, or palynology, is still today an important tool that helps to reconstruct local environment and climatic changes, the stratigraphy of recent deposits, human impact on the landscape, the rise and fall of civilizations and is even used to solve criminal cases.

Bibliography:

BLYTT, A. (1876): Essay on the Immigration of the Norwegian Flora during Alternating Rainy and Dry Periods. Alb. Cammermeyer, Christiana, Oslo, Norway: 89
HILGEN, F.J. (2010): Astronomical dating in the 19th century. Earth-Science Reviews 98: 65-80
POST, V. L. (1944): The Prospect for Pollen Analysis in the Study of the Earth´s Climate History. New Phytologist Vol. 45: 198-203

The Great Leap Forward and soil erosion

"dal letame nascono i fiori,
dai diamanti non nasce niente"
"from dirt flowers are born,
from diamonds nothing comes"
"Via del Campo" by Fabrizio de André (Italian poet-musician)

In Geology soils are defined as the uppermost layer or substratum of earth, it supports most of the plant life and is therefore also essential for all heterotrophic life. Soils are the results of complex interactions between the biosphere-atmosphere-lithosphere and hydrosphere over long periods of time - oversimplifying the remains of the weathering of rocks enriched by organic debris.

Soil degradation and erosion was and still is one of the major threats to soil quality and function. Erosion is a natural process; however human influence and mismanagement can significantly increase the velocity and extent of this process. Unprotected soil can be rapidly eroded by wind or washed away by running water - logging, overexploitation, monocultures can damage, even destroy the plant cover protecting the soil. Irrigation and processing can condense the soil or modify its chemistry. The collapse of many civilisations in the past was triggered by the erosion and degradation of soil, followed by decrease in the agricultural production and widespread famine and death. Even in the 20th century humans - mainly politicians - made such errors leading to terrible consequences for the entire population.

The areas of China covered with Loess are characterized by very fertile soils. For millennia such soils were cultivated by farmers. However, this yellowish, fine-grained, carbonate-rich aeolian sediment is very vulnerable to erosion by wind and water.
Near the small village of Westeregeln (Thuringia, Germany) past quarrying activity has exposed Mesozoic gypsum and limestone formations, covered by Pleistocene sediments. The uppermost part of the stratigraphy is represented by a postglacial soil developed on Loess - a sediment deposited during the last glacial period. Note the secondary infillings of the burrows of animals and the different colors of the horizons of the soil.

The rise of the communistic party under the leadership of Mao Zedong in China after 1966 had a profound impact on society and the environment - it caused one of the greatest humanitarian catastrophes in modern times and effects are still visible in modern China. Inspired by the apparent success of the U.d.S.S.R. under Stalin the Chinese party intended to transform in only few years the rural agriculture economy into a socialistic power - The Great Leap Forward - (as envisaged in the propaganda poster at the top of this post, displaying the production of grains skyrocketing) - following a strange mix of science, personal opinions and pseudoscientific claims, like these formulated by Trofim Denissowitsch Lyssenko, one of the leading agriculture scientists of Stalin's regime.

A preliminary 5-year plan was adopted in the years 1953 to 1957, consisting of a complex pattern of logging and reforestation - in only few months estimated 10% of China's forests were transformed into farmland. To increase the production of iron simple backyard furnaces were constructed, the increased demand for fire-wood led to an even faster deforestation and subsequent soil erosion. Heavy equipment, as used on the cultivated fields in the Russian plains, lead to tillage erosion on the slopes and in the soft soils of the Loess Plateau.

In the years 1957/1958 a second, even more ambitious plan for the next 12-years was adopted - with even more catastrophic effects.
Farmers should plant 12 to 15 million plants per hectare instead of the previous 1.5 million, Mao thought that plants would grow better in a large collective - as results of this overexploitation and the concurrence for light and nourishments most plants in fact died. Plant species ill-suited to the local soils and climate were planted on large areas - especially maize (Zea mays mays). The dense root system of this grass species tends to seal off the soil, water can no longer infiltrate and the upper part of the soils get cloaked by mud particles, limiting the diffusion of oxygen into the soil and finally heavily damaging the growing plants.
The construction of dams and canals modified the catchments of rivers and the hydrology of entire regions; this lead to widespread erosion of the fertile soil and reservoirs became clogged with sediments and could no longer provide water.

The resulting decrease in agricultural production lead to a terrible period of starvation in the years 1958 to 1961, estimated 40 to 30 million people died in this period. Extremist Mao Zedong and many of the leadership of the communistic party ignored, however, the facts and affirmed instead that the famine was the result of saboteurs or opposing political forces - a fiercely which-hunt to find a scapegoat was initiated. Only in 1962, the reformations were taken back and the situation improved.
Despite the disastrous results other communistic countries, like Cambodia, Ethiopia and North Korea, adopted questionable agricultural methods during the 20th century, with similar results.

Soil degradation and erosion are still major problems in modern China., Nineteen percent of the area of the country is still affected, but also in many other industrialized countries of the world soil has become a valuable resource on the global market. China and India are buying or renting large areas in underdeveloped countries. This solution is however problematic, considering that the food production in many of the involved countries is not capable to sustain even their own population.

China´s today politics is an example of a conflict of interests similar to all developed countries. It invests in reforestation, conservation areas and environmental protection. However, at the same time, industry and the increasing population demands for further land use and resources.


BORK, H.-R. (2006): Landschaften der Erde unter dem Einfluss des Menschen. Wissenschaftliche Buchgesellschaft, Darmstadt: 207

Enter the Coal Swamp Forest: The giant Lycopsid

"The "coal swamp" is one of the most powerful images in palaeontology. Dense, dark, and damp populated by strange trees, giant dragonflies, and sluggish tetrapods resting on rotting logs -a diorama can be found in almost every museum and is short-hand for the Carboniferous tropics. However appealing, this visual representation of the coal-swamp forest, based on analogy with modern tropical rainforests, is largely inaccurate."

W.A. DIMICHELE (2001): "Paleobiology II."

Fig.1. Illustration by Zdenek Burian for the book "Prehistoric Animals" (1956) showing a Carboniferous landscape - note in the background a dense forest of tree like lycopsids of the genus Lepidodendron.

Unlike modern forests, that are dominated by two large plant classes (tropical forests for example by angiosperms and boreal forests by gymnosperms) or by only a few species, the Carboniferous forest was composed of at least four classes and more than 10 orders of plants - with strikingly different morphology and ecology. The "Coal swamp" has therefore no modern analogy, despite the classic iconography is inspired mostly by modern Cypress swamps found for example in the modern Everglades of Florida.
Modern swamps and mires are characterized by a spatially gradient in nutrients and hydrology, these factors change also over time. The groundwater table is the dominant factor affecting the plant assemblage. Various fossil evidences show that the water level of the Coal swamp also strongly varied in time: preserved stumps are signs of an increase in the water table, killing vegetation and preserving it, charcoal layers show a decrease and drying up of the mire. The climate and therefore the environment of the Carboniferous swamp was not so monotonous as depicted in our imagination - there were phases of inundation and phases of drought.

Fig.2. Sketch of the outcrop in Victoria Park (Glasgow) preserving various stumps of Stigmaria (Lepidodendron) by Chris Meadows (1880).

The clubmosses, Class Lycopsida (or Lycopodiopsida) appear today as morphologically simple herbaceous plants, but are a very honourable and old group; fossil specimens reach back to the time when the first organisms colonized the dry land during the Silurian. Today more than 1.100 species are described, very similar in their basic morphology: from a horizontally creeping rhizome vertical branches grow straight upwards; these branches support the sporangia (spore producing organs) and are covered by small scaly leaves.

Fig.3. The modern clubmoss Huperzia selago.

Fig.4. ... and the spikemoss Selaginella helvetica.

However during the Carboniferous several lycopsid groups achieved giant size and developed very different shapes - the dominant "tree" seen in most of the reconstructions of the Carboniferous Coal swamps is a giant lycopsid.
The genus Lepidodendron (or "scale tree"), a 35m high tree like lycopsid, was known for almost 200 years from the imprints of the "bark", showing a typical regular patter with the scars of the single leaves. Despite its common appearance in most of the reconstructed landscapes of the Carboniferous epoch, Lepidodendron was surely limited in his geographical and temporal range. The plant was adapted to wet conditions, water transported also it spores - however such habitats were not characteristic for the entire Carboniferous, during the late Carboniferous the climate became drier and the genus Lepidodendron was replaced soon by smaller lycopsid genera, reaching almost 1 meter in height.

Today various different fossils were merged together to reconstruct the morphology of a Lepidodendron tree: The lower part of the stem, referred in the past as Knorria, was smooth, only the upper part, referred as Lepidophloios, was covered with small, needle shaped leaves (Lepidophylloides) similar to the branches of modern lycopsids. An important difference to modern lycopsids was the position of the sporangia - collocated at the end of dichotomously branching twigs and similar to a cone, known previously as the fossil genus Lepidostrobus.

However this reconstruction similar to a modern tree with stem and branches is valid only for a short phase of reproduction of the plant, when the organism finally produces a terminal sporangium. Until this phase it is more plausible assuming that Lepidodendron resembled much more a simple, unbranched lycopsid, forming very open forests with scattered small and large individuals.

Fig.5. Schematic reconstruction of Lepidodendron as adult, fertile individual and younger individual lacking branches with spore cones, probably the usual habit to be spotted in the Carboniferous forest. Depending from author and reconstruction method, the branches of the mature plant were displayed as standing upright or sag to the ground. The scale tree is named after the typical structure preserved on the bark - the leaf cushion - structure that supported small, needle like leaves covering the upper part of the plant.

To be continued...

Bibliography:

BRIGGS, D.E.G. & CROWTHER, P.R. (2003): Palaeobiology II. Blackwell Publishing: 583
SPINAR, Z.V. (1976): Quando l´uomo non c´era. Fratelli Fabbri Editori, Milano: 228
WILLIS, K.J. & McELWAIN, J.C. (2002): The evolution of plants. Oxford University Press - Oxford: 378

Plant Taphonomy

Plants can be preserved in the geologic record in various ways: as a mould, as compression/impression fossils, as permineralized fossils and even as unaltered plant remains.

- A compression fossil forms simply by plant remains that became embedded and buried by accumulating sediments. The water is squeezed out and the plant flattened. The original organic can be conserved as thin carbonaceous film forming a silhouette of the original plant or lost completely. In this case the rocks preserve only an impression of the former plant. This impression can be refilled by minerals, forming a "cast" of the original plant. In this kind of fossil the general morphology of the plant can be studied.

Fig.1. Rhacopteris asphlenites from the Carboniferous showing preservation by compression (carbonaceous film) and impression.

Fig.2. Fossil Ficus species from the Eocene lagerstätte of Bolca (Italy), here the original plant tissue was replaced by reddish coloured iron oxides.

Fig.3. Asplenium scolopendrium preserved as imprint in Holocene travertine. If such an imprint is refilled with other minerals a three-dimensional cast of the former plant can be formed.

Fig.4. Cast of a piece of wood composed of silica discovered in a volcanic tuff from the Permian.

Coal is a compression fossil in which the organic substance is enriched in carbon and depleted by other volatile elements like hydrogen or nitrogen in various degrees, when plant remains are still visible it is classified as Lignite, when completely homogenized it is called Anthracite.

Fig.5. Example of coal in thin section, showing collapsed Megaspores, amalgamated plant fragments (Fusinite) and homogenous matrix.

- Permineralization is the classic petrifaction; the organic substance of the tissue is replaced by minerals deposited from percolating fluids. These minerals can be calcium carbonate, iron oxides or hydroxides, iron sulphides, silica, or - in cold environments - also ice. This kind of fossil is preserved undeformed and can deliver information about the three-dimensional structure of the plant.

Fig.6. Polished transverse section of a tree fern of the genus Psaronius, showing the structure of the false trunk, composed of roots growing together.

- Some tissues of plants are exceptional stable, like the cuticle covering the surface of a plant, or the Sporopollenin, organic substance forming the hull of spores and pollen grains. These components can be conserved, under the right conditions, unaltered for million of years.
The epidermis and surface of plants possesses distinctive characters useful for taxonomic classification, like for example the superficial cell pattern, or the shape and distribution of the stomata, papillae and glands, spores and pollen grains also possess very distinctive surface patterns.
If microbial activity is inhibited or oxygen not available also the less resistant plant tissues can be embedded in sediments and conserved unaltered. The most important fossils of this kind were recovered from lake sediments, amber and packrat middens.
In lakes due differences in temperature and density of the water the bottom can became depleted of oxygen, preventing organisms to colonize and feed on organic material, also a rapid sedimentation quickly covers the plant remains, optimal conditions for fossilisation. Some lakes, especially swamps, contain also high concentration of organic substances, washed into the water from the land, that kill microbes, impregnate and preserve organic material.
Amber preserves organic material in various ways, when hardened resin has a limited mechanical protection effect, it protects the organism from scavengers and weather and isolates it from oxygen, which can oxidize the organic material or enable bacteria to destroy the inclusion. Resins of plants possess also an antibiotic effect and therefore kills microorganisms - in Egypt this effect was used to sterilize artificial mummies. The content of "sugar" in the resin probably also draw moisture out of the tissue, preventing further microbial activity.

Fig.7. Example of plant tissue preserved in amber, trichomes of a oak tree florescence.

Despite these cases it is very rare that a plant becomes completely fossilized - especially large plants, like trees, tend to be embedded in sediments only in fragments.
This causes major problems; some plant tissue tends to be overrepresented in the fossil record. For example wood and bark are more resistant than non lignified plant tissue and will became fossilized more easily - this can produce the effect that herbaceous plants are generally underrepresented in the geologic record. In contrast leaves or pollen are produced in great quantities even by a single plant, this can produce a bias of the stratigraphic record to some species of plants.

Fragmented plant remains also make it difficult to reconstruct the overall morphology of a single plant species, and worse, cause a proliferation of artificial plant species. A single plant can produce diverse organs and tissue and therefore fossils - like foliages (which often various shapes and sizes on a single individual), roots, stem, branches, bark, male or female cones in gymnosperms, blossoms and fruits in angiosperms.
Many fossils were found dissociated and attributed to individual species, only subsequent discoveries revealed in some cases which tissues belong together, forming a single plant species. For example the extinct Lepidodendron-tree, related to the modern Lycopsids, was reconstructed by merging together at least seven single "species" (Stigmaria - roots, Knorria - bark, Lepidophloios - bark, Lepidostrobus - cone, Lepidophylloides - leaf etc.) based on the single organs or tissues of the former plant.

Also fragments of plants not necessarily represent a real association of plants - a biocenosis. Plants or parts of them can be transported by wind (for example leaves) or by water (for example wood), therefore plant species of various regions and ecosystems can became deposited and mixed together in a sedimentary basin. This false association of plants is called taphocenosis - a death assemblage.

Fig.8-10. Litter in a modern forest, showing fragmentation of a single or few tree species in various "morphospecies" based on the single plant organs, like cones, needles, branches, wood, intermixing with herbaceous plants and even stranger live forms as fungi are, forming possibly in the geologic record a future taphocenosis.

Many reconstructions of former landscapes show an incredible biodiversity of plants mixed chaotically together, but observing modern landscapes we note that most plant associations are characterized by few plant species, dominating a specific habitat and that there is a continuum of various habitats following in succession. We will, for example, find a specific plant-association on a shore, and a specific plant-association on the dry highland.

Fig.11. "Coal swamp" as imagined by Z. Burian in 1972 (found in SPINAR 1976).

A concrete example of the biased reconstruction of the former vegetation is the classic and in textbooks ubiquitous "coal swamp", that I will discuss soon…

Bibliography:

BENTON, M.J. & HARPER, D.A.T. (2009): Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell Publication: 592
WILLIS, K.J. & McELWAIN, J.C. (2002): The evolution of plants. Oxford University Press - Oxford: 378
SPINAR, Z.V. (1976): Quando l´uomo non c´era. Fratelli Fabbri Editori, Milano: 228

Online Resources:

ARENS, N.C. et al. (1998): Plant Fossils and Their Preservation. (Accessed 18.06.2011)

A very short history of Paleobotany

"There are so many plants on the earth, that there is a danger to thinking them trivial...[]...What a marvellous cooperative arrangement, plants and animals each using the others waste gases, the whole circle powered by abundant sun light. But there would be carbon dioxide in the air, even if there were no animals. We need the plants much more then they need us."

Carl Sagan in "Cosmos", ep. 2 "One Voice in the Cosmic Fugue"

Fig.1. A modern fern blade compared to a fossil of the Carboniferous (almost 300 Ma) from Southern France.

Plant fossils are possibly the first petrifactions recognized as the remains of once living organisms - many fossils species still resemble extant species and it was therefore straightforward to compare and match them together.
Already in 1699 the English naturalist Edward Lhwyd (ca. 1660-1709) depicted various fossil plants in his catalogue of the English fossil titled "Lithophilacii Britannica ichnographia". Also Carl von Linné (1707-1778) studied and published some notes about plant fossils hosted in the cabinets of curiosities of Swedish noblemen.
The oldest publication dealing almost exclusively with plant fossils is however the "Herbarium diluvianum" by the Swiss scholar Johann Jakob Scheuchzer, published in various edition in 1709 and 1723.

Fig.2. Plate from J.J. Scheuchzer´s opus magnus "Herbarium diluvianum" (1709-1723).

Scheuchzer published his studies and observation of many fossils plants coming from the English Carboniferous, the Permian of Germany and the Swiss Cainozoic, including plant casts found in travertine. He argued that these imprints were once living plants like recent ones that became buried in sediments by the biblical flood. His was so convinced of the similarities of fossil to modern plants that he even tried by phenology to determinate the month when the flood occurred.

Unlike bones, shells and ammonites, collected and studied avidly by amateurs, paleobotany was since its early origins dominated by professional scholars - the identification of fossil plants requires apparently more skills and profound knowledge in modern botany, abilities or interests not encouraged enough in all times?

Fig.3. Plate from Caspar von Sternberg´s "Versuch einer geognostisch-botanischen Darstellung der Flora der Vorwelt" (Attempt to a geognostic-botanical representation of the flora of the ancient world) (1820-1838), showing the "bark" of an extinct Lepidodendron-"tree" related to the modern Lycopsids.

In 1804 the German Geologist and Palaeontologist Ernst Freiherr von Schlotheim (1764-1832) published a description of Permian plants "Beschreibung merkwürdiger Kräuter-Abdrücke und Pflanzen-Versteinerungen" (Description of curious imprints of herbs and plant-petrifactions), where he adopted on the fossil plants the systematic-system developed by Linné and compared their ecology to modern plants, establishing finally the principles of modern Paleobotany.
Important contributions to botany of fossil plants and pollen grains and there use in stratigraphy came from the French geologist Alexandre Brongniart (1801-1876 - with Cuvier one of the first to draw startigraphic profiles in Europe) with his "Histoire des vegetaux fossiles, ou recherches botaniques et geologiques sur les vegetaux renfermes dans les diverses couches de globe"(1828-1838).

Fig.4. Plate from A. Brongniart´s "Histoire des vegetaux fossiles" (History of fossil plants) (1828-1838).

After the first general works on paleobotany surprisingly fast more specific research on single formations, geographical areas or plant groups was published, for example the German Heinrich Robert Göppert (1800-1884) studied the Tertiary brown coal deposits and the Austrian physician Franz Unger (1800-1870), after becoming professor for paleobotany in Graz and Vienna, studied the coal deposits of Styria. In his "Iconographia plantarum fossilium" he compares the fossil flora with the forests of Indonesia, South America, the Caribbean and Mexico, recognizing that the plants indeed can be used to reconstruct the climate of a tropical past - it is no wonder that geologist Henry de la Beche (1796 -1855) collocated "Professor Ichthyosaurus" in a tropical jungle with palm-trees and gigantic ferns.

Fig.5. Plate from H.R.Göppert´s "Die Tertiärflora auf der Insel Java" (Tertiary flora of Java) (1854).

To be continue...

Online Resources:

ROBERTS, B.F.; SHARPE, R. & WATT, H. (): Edward Lhwyd. (Accessed 09.06.2011)