CHAPTER X 



SUBKINGDOM SPERMATOPHYTA (SEED-PLANTS) 

 CLASS I. GYMXOSPERM^E 



HETEROSPORT arose independently in all of the classes of Pterido- 

 phytes. Two types may be recognized, that in which the contents 

 of the germinating macrospore divide at once by cell-walls, as in Mar- 

 silia and Salvinia, and that in which there is a repeated division of 

 the nucleus, before cell-formation begins. The latter occurs in 

 Isoetes and Selaginella, and much more nearly resembles the condi- 

 tion found among the Spermatophytes or Flowering-plants, also 

 known as " Phanerogams." 



In Selaginella, the growth of the gametophyte within the macro- 

 spore begins before the latter has reached its full size, and while it 

 is still retained within the sporangium, whose wall-cells remain active 

 until the growth of the macrospore is complete, and the develop- 

 ment of the latter is largely due to material conveyed to it through 

 the agency of the outer sporangial cells. In both the early devel- 

 opment of the gametophyte and the participation of the outer 

 sporangial tissue in the growth of the spore and the contained 

 gametophyte, Selaginella resembles more nearly than any other 

 living Pteridophyte, the condition found in the Spermatophytes, or 

 Seed-plants. 



The Seed 



In the Spermatophytes, as in Selaginella, the germination of the 

 macrospore begins before it is full grown ; but unlike Selaginella, the 

 ripe macrospore is not expelled from the sporangium, but remains 

 permanently within it, and usually, although not always, fertilization 

 of the archegonium is effected while the sporangium is still attached 

 to the sporophyte. After fertilization has been effected, the outer 

 tissues of the macrosporangium harden, and form a protective cover- 

 ing for the enclosed macrospore, within which He the gametophyte 

 and embryo-sporophyte. Sooner or later, the sporangium falls away, 

 and the collective structure, the sporangium, with the enclosed gam- 

 etophyte and embryo, is known as a Seed. This modified macrospo- 

 rangium is the characteristic of all Spermatophytes, but as it is 



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