CHAPTEE 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 sporopliyte 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 heterospory and reduction of gameto- 

 pliytes. — 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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