GYMNOSPERMS 



323 



The female gametophyte (Fig. 187, B) is forme.d within the 

 megaspore, which enlarges as the gametophyte grows, until it 

 comes to occupy most of the space within the megaspore wall, 

 only a remnant of the sporangial tissue being left at the micro- 

 pylar end of the sporangium. At the micropylar end of the game- 

 tophyte from three to five archegonia are formed. The male 

 gametophyte at first consists of a single gametophyte cell and of 



Miorosporcmffia 

 Microsporangvutm 



-Microspore 



'ametophyte 

 Generative cell 

 Tube nucleus 



Gametophyte 



Stalk cell. 

 Sperm mother 

 cells 



Pollen tube 



FIG. 187. Habit and reproductive structures of a Florida cycad, Zamia 



A, plant of Zamia bearing a female strobilus; 13, a, megasporophyll, sporangia, ard 



spores; &, megasporangium (ovule) with gametophyte, archegonia, pollen grains, 



and pollen tubes ; (7, a, microsporophyll and sporangia ; d, e, two stages in the 



germination of the microspore and in the production of the male gametophyte 



Fleshy layer 

 -^Integument 

 Pollen chamber 

 Micropyle 



an antheridial cell, called the generative cell (Fig. 187, (7, d). The 

 generative cell then divides and there are ultimately formed a 

 stalk cell and two sperm mother cells, each of which then de- 

 velops a motile sperm (Fig. 187, (7, , /). Pollination takes 

 place when the sporophylls of the female strobili separate and the 

 microspores are borne to them by air currents. The microspores 

 are drawn into the micropyle by a secretion which contracts with 

 drying and carries the spores into a chamber (called the pollen 

 chamber) developed in the sporangial tissue at the base of the 

 micropyle (Fig. 187, B). The pollen tubes developed by the 



