CIIAPTEE XXIII 



SPERMATOPHYTES : GYMNOSPERMS 



223. Summary from Pteridophytes.— In considering tlie 

 important contributions of Pteridopliytcs to tlie evolution 

 of the plant kingdom the following seem worthy of note : 



(1) Frominenve of sporophijte ami devehipmeut of rascu- 

 Jar 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- 

 ctilar system. This fact is true of the whole group. 



(2) DiffiTentiatiou of spuropliiills. — The appearance of 

 sporojAylls as distinct from foliage leaves, and their or- 

 ganization into the cluster known as the strobilus, are facts 

 of prime importance. This differentiation ajjpears more or 

 less in all the great groups, but the strobilus is distinct only 

 in Horsetails and Club-mosses. 



(3) Introduction of Jietcros^xiri/ and reduction of gameto- 

 plnjtes. — Ileterospory appears independently in all of the 

 three great groups — in the water-ferns among the Fili- 

 cales, in the ancient horsetails among the Equisetalcs, and 

 in Selaginclla and Isoetes among Lycopodiules. All the 

 other Pteridophj'tes, 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 j^roduce tliem. 



223. Summary of the four groups. — It may be well in this 

 connection to give certain prominent characters which will 

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