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 100 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 
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