THE HISTORY OF PLANTS 



409 



prostrate on the exposed muddy shores of ponds and streams. This would 

 have been fatal to most of them; but if some developed a cuticle on the 

 exposed parts and water-absorbent rhizoids on the lower surface, they 

 not only could have survived but could have come to live on permanently 

 moist ground away from bodies of water. A later evolution of vascular 

 tissues would have permitted upward growth of shoots, exposing more 

 surface to sunlight. 



There is one surviving group of plants that remains in a stage which is 

 approximately the same as this early adaptation to land conditions — the 

 bryophytes. The simplest of these are the liverworts (Fig. A. 10), most of 

 which have flat, thin bodies that lie 

 pressed against the surface of the 

 moist soil or rock upon which they 

 grow. Above they are covered with 

 cuticle; below they have many fine 

 hairlike processes that absorb water 

 from the substratum. A few have 

 erect shoots bearing small leaflike 

 structures, but these liverworts are 

 all very small, lacking any trace of a 

 vascular system. The mosses (Figs. 

 17.3 and 4) are more advanced bryo- 

 phytes, with stems and leaflike 

 structures ; but they also are without 

 vascular tissues and in consequence 

 are all small, being unable to trans- 

 port water to any considerable 

 height. Bryophytes bear their spore- 

 producing organs on stalks, for wider 

 dispersal of the spores, but the fact 

 that their sperm cells must swim in water films to reach the eggs has kept 

 them from complete adaptation to land life. The living bryophytes show 

 us what the earliest land plants must have been like, and the group is 

 presumably very ancient, though fossil liverworts and supposed mosses 

 are first known from the Carboniferous. 



The first vascular plants. It is not bryophytes but simple vascular 

 plants, which appear earliest in the fossil record. The oldest are from the 

 mid-Silurian of Australia, but it was in the Devonian period that they 

 first became numerous and varied. The simplest and probably the most 

 ancient were the psilpphytes, which may have been the ancestors of all 

 the later vascular plants. They were slender green stems, a few inches to a 

 foot or more in height, naked or covered with small leaves, and bearing 

 spore cases at their tips. They had no roots but rose from horizontal 



Fig. 26.9. Psilophytes from the Devonian 

 of Scotland, restored. Left, Asteroxylon 

 mackiei; right, Rhynia major. (From Haupt, 

 An Introduction to Botany.) 



