THE GYMNOSPERMS 403 



Seventh. The complete dependence of the gametophytes on the 

 sporophytes, which also was introduced by the higher Pterido- 

 phytes but in the Gymnosperms has been carried farther, the 

 female prothallium never escaping from the ovule or macro- 

 sporangium. The female prothallium, or endosperm, is parasitic 

 in the nucellus of the ovule (macrosporangium) , for when it is 

 mature the nucellus is nearly or quite consumed; only a papery 

 remnant surrounds the endosperm. 



Eighth. The introduction of pollination, necessitated by the 

 fact that the female prothallium does not escape from the ovule 

 and is separated from the outside by the nucellus tissue, i.e., a 

 part of the ovule or macrosporangium. The pollen or micro- 

 spores must be transported to the ovule. 



Ninth. The germination of the tube cell, or wall cell of the 

 antheridium to form the pollen tube, which penetrates the 

 nucellus tissue in the case of the cycads and Gingko to provide 

 nutriment for the development of the motile sperms, which then 

 swim from the pollen chamber into the archegonia, while in the 

 conifers the pollen tube penetrates the nucellus, and obtains 

 nutriment for the same purpose, but makes its way directly into 

 the archegonium into which it then empties the sperm cells. 

 The pollen tube is thus parasitic in the nucellus, for its behavior 

 is like that of a parasitic fungus mycelium, obtaining food from 

 the tissues through which it grows. 



Tenth. This new feature of pollination and growth of the 

 pollen tube is necessitated because of the more highly acquired 

 land habit of the Gymnosperms and the complete dependence of 

 the gametophyte on the sporophyte, while in the Pteridophytes, 

 the gametophytes, either from the first, or as in Selaginella, some 

 time before fertilization, becoming free from the sporophyte are 

 dependent on water as a means of the motile sperms being con- 

 ducted to the archegonia. 



Eleventh. The development of the seed, which is a new struc- 

 ture for the propagation and spread of the plants. It will be 

 recalled that Selaginella almost reached the point of forming 

 seeds. 



