DEVELOPMENT OF PLANTS 357 



giiim, into nourishing cells. This structure of the megasporan- 

 gium is characteristic of all the spermatophyta and the arrange- 

 ment is of decided advantage since it provides the megaspore 

 during its formation and germination with an abundance of 

 food and moisture. 



(b) The Gametophyte of the Cycads. — The megaspore germi- 

 nates as in Selaginella. Numerous free nuclei are first formed 

 that are arranged around the walls of the spore and by the 

 later formation of walls a tissue is developed that gradually 

 fills the enlarging spore. Archegonia now appear at the micro- 

 pylar end of this gametophyte and become sunken in pits, called 

 the archegonial chamber, owing to a continued growth of the 

 adjacent cells of the gametophyte (Fig. 246). While this female 

 gametophyte is very suggestive of the one noted in Selaginella 

 it should be observed that the archegonia are more simple in 

 structure, usually possessing but two neck cells and that the 

 central canal cells of the neck are entirely lacking. The female 

 gamete is also much larger (Fig. 246, 0) than any heretofore 

 noticed and it is nourished by specialized cells, the so-called 

 jacket cells, that surround it and supply it through minute pores 

 with an abundance of food. 



It is evident that the buried position of the archegonia must 

 necessitate some new departure in order to bring the male ga- 

 metes to the archegonia. The microspores have already begun 

 their germination at the time when they are discharged from the 

 sporangium and being very light, they are carried by the winds 

 to strobili containing megasporangia. When the megasporo- 

 phylls are mature, they spread apart slightly, permitting the 

 microspores to rattle down to the sporangia where they are 

 caught, at least in some forms, by a mucilaginous secretion 

 from the micropyle. This substance in drying contracts and 

 pulls the spores in large numbers through the micropyle into 

 a chamber formed in the upper part of the sporangium (Fig. 

 246, p). As has been stated, the microspore begins to germinate 

 in its own sporangium and by the time that it reaches the cavity 

 of the megasporangium its nucleus has already divided twice, 

 forming a rudimentary male gametophyte of three cells that may 



