CHAPTER XXIII 
SPERMATOPHYTES : GYMNOSPERMS 
222. Summary from Pteridophytes. In considering the 
important contributions of Pteridophytes to the evolution 
of the plant kingdom the following seem worthy of note : 
(1) Prominence of sporophyte and development of vascu- 
lar system. This prominence is associated with the display 
of leaves for chlorophyll work, and the leaves necessitate 
the work of conduction, which is arranged for by the vas- 
cular system. This fact is true of the whole group. 
(2) Differentiation of sporophylls. The appearance of 
sporophylls as distinct from foliage leaves, and their or- 
ganization into the cluster known as the strobilus, are facts 
of prime importance. This differentiation appears more or 
less in all the great groups, but the strobilus is distinct only 
in Horsetails and Club-mosses. 
(3) Introduction of lieterospory and reduction of gameto- 
phytes. Heterospory appears independently in all of the 
three great groups in the water-ferns among the Fili- 
cales, in the ancient horsetails among the Equisetales, and 
in Selaginella and Isoetes among Lycopodiales. All the 
other Pteridophytes, and therefore the great majority of 
them, are homosporous. The importance of the appear- 
ance of heterospory lies in the fact that it leads to the 
development of Spermatophytes, and associated with it is 
a great reduction of the gametophytes, which project little, 
if at all, from the spores which produce them. 
223. Summary of the four groups. It may be well in this 
connection to give certain prominent characters which will 
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