410 



THE CHANGING GENERATIONS 



underground stems bearing rhizoids. The shoots had a central strand of 

 xylem surrounded by phloem, and possessed cuticle and stomata. Psilo- 

 phytes seem to have formed meadowlike stands in swampy spots and 

 along stream margins. With them occurred two slightly more advanced 

 types of plants — horsetails, or scouring rushes, with fluted, jointed stems 

 and whorls of leaves or branches at the nodes; and club mosses, or ground 



Fig. 26.10. Restoration of the earliest known forest, at Gilboa, N.Y. (Courtesy New York 

 State Museum.) 



pines (lycopods), having stems densely covered with small pointed leaves 

 and bearing spore cases in the leaf axils. The psilophytes became extinct 

 by the end of the Devonian, but the other two groups prospered and some 

 of their descendants increased greatly in size. 



The first forests. By mid-Devonian times true ferns had appeared; 

 of all the familiar present-day groups of plants these are the oldest. 

 Throughout the late Paleozoic and Mesozoic they formed the under- 

 growth in moist, shady places, just as their modern descendants do. The 

 ferns were spore bearers then as now, but some unknown member of the 

 group gave rise to the first true seed plants. These were the seed ferns 



