218 



MORPHOLOG Y. 



In comparing the female prothallium of the gymnosperms with that of the 

 fern group we see a remarkable change has taken place. The female pro- 

 thallium of the gymno- 

 Ex ^ r> sperms is very much 



A I ! reduced in size. Espe- 



Ei > 



dally, it no longer leads 



an independent existence 

 from the sporophyte, as 

 is the case with nearly 

 all the fern group. It 

 remains enclosed within 

 the macrosporangium (in 

 cycas if not fertilized it 

 sometimes grows outside 

 of the macrosporangium 

 and becomes green), and 

 derives its nourishment 

 through it from the sporo- 

 phyte, to which the latter 

 remains organically con- 

 nected. This condition 

 of the female prothallium 

 of the gymnosperms 

 necessitated a special 

 adaptation of the male 

 P r <>thallium in order that 

 the sperm cells may reach 

 and fertilize the egg cell. 



Fig. 2 



Gingko biloba. A, mature pollen grain; /?, germinating 

 pollen grain, the branched tube entering among the cells 

 of the nucellus; Ex, exine (qjjter wall of spore); P lt pro- 

 thallial cell ; T^, antheridial cell (divides later to form stalk 

 cell and generative cell) ; f\, vegetative cell ; l^a, vacuoles ; 

 Nc, nucellus. (After drawings by Hirase and Ikeno.) 



Fig. 287. 



Gingko biloba, diagrammatic representation of the relation of pollen tube to the arche- 

 gonium in the end of the nucellus. pt, pollen tube ; 0, archegonium. (After drawing by 

 Hirase and Ikeno.) 



437. Gymnosperms are naked seed plants. The pine, as we have seen, 

 has naked seeds. That is, the seeds are not enclosed within the carpel, but 



