January, 



1938 



EVOLUTION 



Page Five 



Fossil Plants and Evolution 



By WM. C. DARRAH 

 Reseaich Curator of Paleobotany, Botanical Museum, Harvard University 



FEW fossil discoveries ha\e done more to complete our 

 knowledge of the continuous evolution of the plant 

 kmgdom than the discovery of the simplest and oldest 

 land plants. 



Almost eighty years ago — in 1859 — Sir William Dawson 

 discovered an extremely simple-looking plant in the Dev- 

 onian rocks of Gaspe. This plant, to which he gave the 

 name Psilophyton, had little more 

 than the bare essentials for existence 

 on land. The shoot, scarcely a foot 

 in height, had a bit of underground 

 stem without roots and a series of 

 forking branches without lea\'es, which 

 bore small terminal spore cases crowd- 

 ed with small spores. 



For more than fifty years Dawson's 

 plant was considered to be the inac- 

 curate restoration of an imaginative 

 paleontologist, without scientific inter- 

 est or value. 



During the closing years of the 

 World War two British paleobotanists, 

 Robert Kidston and Willi;mi Lang, 

 began a series of investigations on the 

 Rhynie Chert, which occurs in Aber- 

 deenshire, Scotland. The Rhynie 

 Chert is a flint-like rock formed by 

 the accumulation of silica from silica- 

 bearing ground water. Fortunately the 

 silica accumulated in a peatbed crowd- 

 ed with the successive generations ol 

 small plants which lived in the swamp 

 more than 300 million years ago. 



It so happened that the silica in 

 solution acted as a preservative against 

 decay, and, as a result, not only fossil- 

 ized the plant debris but also preserved 

 the microscopical cells and tissues of 

 the organisms. The life history of 

 these early Devonian plants is therefore 

 known in great detail and with amaz- 

 ing completeness. 



Immediately it was recognized that Rhynia, the most 

 abundant of these Scotch plants, was a close relative ot 

 Dawson's Psilophyton which had been disregarded tor 

 half a century. 



Botanists enthusiastically received the results of these 

 investigations on early Devonian plants. Their salient fea 

 tures are essential to understanding the evolution of all 

 higher plant groups. These small plants are grouped to- 

 gether into one "order" technically termed the Psilophytales. 

 They not only have the extremely simple external appear- 

 ance of Psilophyton as described by Dawson, but are accom- 

 panied by an equally simple and undifferentiated internal 

 organization. The stem is supported by a slender central 

 cylinder of wood. 



The epidermis of the stem contains many breathing 

 organs or stomata showing that the plant was able to live 

 on dry land and manufacture its own food, despite the fact 

 that it bore no leaves which, in more evolved plants, nor- 



The Earliest Land Plant 



Psilophyton 

 Reconstructed by Dawson 



mally carry on food production. This group of ancient 

 plants had acquired little more than the bare necessities 

 of a land existence, eating to live and living to reproduce 

 their kind. There were no embellishments. 



It is not quite accurate to speak of these plants as the 

 earliest plants although they are almost as simple as bot- 

 anists can conceive. Within the past two years, very similar 

 but still more ancient plants have been 

 discovered in the Silurian rocks ot 

 Victoria, Australia. 



Through these remarkable discover- 

 ies we can study the anatomy and 

 reproductive structures of long-extinct 

 plants in detail. 



All of the older Devonian plants 

 reproduced by spores — small single- 

 celled structures capable of being dis- 

 tributed in wind or water and living 

 independently of the parent plant for 

 long periods of time. Spores are so 

 resistent to decay and destruction that 

 they are familiar fossils, despite their 

 minute (usually microscopic) size. 

 For example, a Devonian plant named 

 Sporocarpon from the vicinity of Dela- 

 ware, Ohio, is known in its late em- 

 bryological stages. In figure two, the 

 photomicrograph shows a cluster ot 

 four spores, or tetrad, which have 

 grown from a spore-mother cell. These 

 spores are 280 million years old. 



Spores are sometimes so abundant 

 and accumulate in such great quan- 

 tities that they form entire coal beds. 

 So-called cannel coals and many oil- 

 shales are largely composed of isolated 

 spores. 



These simple ancestral spore-bearing 

 plants gave rise to greater and more 

 diversified plants. Then development 

 of wood soon made increased size pos- 

 sible. The great Carboniferous spore- 

 bearers, such as Lepidodendron, attained an over-all height 

 of 100 feet even with the obstacle of a scant supply ot 

 wood. In these trees, the progenitors of our existing lowly 

 ground pine, the fleshy cortex was so thick that German 

 botanists have named them "rindenbaume" or rind-trees. 

 Nevertheless, spore-bearers are limited in their potential- 

 ities. A large number of spores must be produced so that 

 the successful germination of a few is assured. This is 

 wasteful and new races of plants gradually produced fewer 

 spores of two kinds, male and female, which were supplied 

 with mechanisms for survival during unfavorable periods. 

 The culmination of this trend was the evolution of the 

 pollen (male) and the ovule (female). The fertilized 

 ovule grows into the familiar seed. 



One of great gaps in the story of plant evolution at the 

 turn of the twentieth century was the transition from the 

 higher spore-bearing plants, such as the ferns, to the lower 

 seed-plants, in cycads and conifers, our cone-bearing trees. 



