APPENDIX A 



613 



This group is of peculiar interest because of the fact that its reproductive proc- 

 esses show an advance over Lycopodium in the direction of the higher plants. 

 Instead of all its spores being alike (homosporous condition) as in Lycopodium, 

 they are of two sorts (microspore and megaspore) as in all the higher forms, in- 

 cluding the seed plants. The microspore produces a minute male gametophyte 

 that remains completely enclosed in the spore wall; the megaspore germinates 

 into a much larger but still minute female gametophyte that remains within 

 the megasporangium. The microspores, with their contained male gametophytes, 



Fig. A. 14. The extinct Carboniferous horsetail Calamites, which grew to a height of 50 feet 

 and a diameter of 2 feet. The leaves were borne in whorls of 6 to 20, encircling the branches. 

 A, base of plant showing whorled branching. B, leaves. C, section of cone with (s) sporan- 

 gium and (b) enclosing bract. D, part of stem. (Courtesy General Biological Supply House, 

 Inc.) 



are shed in large numbers. In the presence of moisture they liberate swimming 

 sperms, some of which find their way to the egg cells in the gametophytes and 

 fertilize them. Here is obviously the first stage in the mechanism that has reached 

 its culminating development in the spermatophytes. A similar life cycle occurs in 

 the peculiar genus Isoetes (order Isoetales), which comprises small grasslike 

 plants that grow partly submerged in swampy land and pond margins; each of the 

 slender leaves bears a sporangium near the base, and the megaspore is large and 

 seedlike. 



The lycopods have had a long geological history. The present-day orders are 

 the descendants of the Paleozoic *Lepidodendrales, which are first known from 

 the Devonian. They became the dominant plants of the late Paleozoic era and 

 formed a large proportion of the trees of the coal forests. Some of these trees were 



