PHYLOGENY 



luin Pteropsida, including ferns, conifers and their allies, and the flowering 

 plants. How the vascular plants arose from their algal ancestors is not 

 known, and the fossil record throws very little light upon this important 

 question. Nearly sixty years ago, the French botanist Lignier published 

 a highly speculative theory upon this subject, and in the meantime such 

 fossils as have been discovered have been consistent with his theory, even 

 if it cannot be said that they proved it. According to Lignier's theory, the 

 ancestor of the vascular plants must have been a green alga characterized 

 by branching filaments, and this plant must have been a tide-flat dweller. 

 As the land was elevated, tide pools became isolated and then dried up. 

 Much of the flora became extinct. But if one or more of the branches of 

 such an alga should penetrate the ground, it might become transformed into 

 a root system capable of supplving the plant with water and minerals. 

 Some of the branchings might then straighten out, leaving a main stem 

 or trunk with branches. Because the entire plant is no longer immersed 

 in water, a conducting system is now necessary, and only those plants 

 which develop one can survive. Thus the stems become thickened, and 

 the ends flatten out to form specialized organs for photosynthesis, the 

 leaves. Now the conducting system must operate in both directions, carry- 

 ing water and salts up from the roots and carrying organic compounds 

 down from the leaves. Finally, only those plants which developed a cuticle 

 over the aerial parts could escape extinction by drying. While critical 

 proof of this theory is still lacking, evidence accumulated in the past sixty 

 years is consistent with it, and no more probable theory has vet been 

 proposed. 



Subphylum Psilopsida. The subphylum Psilopsida includes two orders, 

 and perhaps the sharpest distinction between them is simply one of time: 

 the Psilophytales are known only from rocks of Silurian and Devonian 

 age, while the Psilotales are represented by two living genera, Psilotiini 

 and Tmesipferus. A series of fossils connecting these two orders is lacking, 

 and it has been suggested by some botanists that they are actually un- 

 related. But the similarities of the plants of these two orders are so close 

 that it seems very improbable that they could be coincidental. The first 

 discovered member of the group, Psilopliifton, was described in 1858 by 

 Sir William Dawson on the basis of fossils from the Devonian of Canada. 

 The plant looked much like a small, r()otl(\ss, leafless shrub (Figure 37). 

 It did not fit into the botanical classifications then in use, and Dawson's 

 discovery was ignored or disparaged. But in 1917 Kidston and Lang dis- 

 covered three genera of similar plants in a silicified bog of Devonian age 

 at Rhynie, Scotland. The preservation of the Rhynie fossils was so good 

 that the cell walls can be seen in thin sections. The fossils are complete 

 and abundant. In the meantime, at least three more g(Miera of psilophvtes 

 have bei'u discovered, so that there is now a wide variety of material 

 available- lor study of this very primitive- group which mav be the con- 

 necting link between the green algae and the vascular [)lants. 



P.sil()})luilon was a small plant growing to a height of about three feet. 

 It consisted almost entirely of a green, branched stem. True leaves were 

 absent, but were- suggested by numerous small spine-like projections of 



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