CHAPTER III — PTERIDOPHYTES 



Introductory. — The gap between bryophytes and pteridophytes is 

 perhaps the greatest in the plant kingdom. To pass from the leafless, 

 dependent sporophyte of bryophytes to the leafy, independent, vascu- 

 lar, root-bearing sporophyte of pteridophytes is a very sudden and 

 complete change. One of the great problems in the evolution of 

 plants is to explain how the leafless sporophyte became a leafy one ; 

 and a part of the problem is to discover the most primitive sporophyte 

 among pteridophytes, concerning which there is great diversity of 

 opinion. For convenience of presentation, the sequence of groups 

 suggested by Bower will be used. 



(i) Lycopodiales 



General character. — The club mosses are widely distributed and 

 comprise about one eighth of the living pteridophytes. The group 

 includes four living genera and also numerous extinct forms, among 

 which are some of the oldest known vascular plants. The three genera 

 Lycopodium, Phylloglossum, and Selaginella are evidently closely 

 related, forming a very natural group, while the fourth genus, Isoetes, 

 has given rise to much discussion as to its affinities. 



Lycopodium 



General character. — This genus, comprising about ioo living species, 

 is in all probability one of the oldest living genera of vascular plants, 

 and possibly is represented in the Paleozoic. It deserves a somewhat 

 full description, as it is possibly the best living representative of the 

 earliest forms of vascular plants. 



Sporophyte. — The sporophyte in its simplest form is a simple stem 

 covered with very numerous small leaves, and on the upper side of 

 each leaf there is a single large sporangium (fig. 265). Leaves bearing 

 sporangia are called sporophylls, and therefore this simplest vascular 

 sporophyte is a simple leafy stem, with every leaf a sporophyll. An 

 assemblage of sporophylls is a strobilus, and therefore this primitive 



