UPERMATOPHYTA 



325 



Male Gametophyte. The first division of the microspore nucleus 

 results in the formation of a small persistent prothallial cell that is cut off 

 close to the microspore wall. The larger cell divides again to form the 

 generative and tnbe cells, and in this condition the pollen grain is shed 

 (Fig. 276.4, B). The pollen is transported by wind to the ovulate cone, 

 whose sporophylls separate slightly at the time of pollination. A group 



/ 



A B 



Fig. 273. Megasporophylls of cycads. A, Cycas revoluta, showing pinnate blade with 

 conspicuous leaflets; B, Cycas circinalis, the leaflets reduced to teeth; C, Macrozamia 

 denisonii, side view of sporophyll, the blade reduced to a spine; D, Zamia floridana, with 

 peltate sporophyll; A, B, C, two-fifths natural size; D, four-fifths natural size. 



of cells at the apex of the nucellus break down and form a droplet of liquid 

 that exudes through the micropyle and to which some of the pollen grains 

 adhere. As the droplet dries, the grains are drawn down into the pollen 

 chamber formed by the disintegration of the cells that produced the drop- 

 let. Then a pollen tube develops from each pollen grain, growing lat- 

 erally into the nucellus. Its basal end advances downward, carrying the 

 prothallial and generative cells with it. In the cycads the pollen tube is 

 an absorbing organ, obtaining nourishment from the nucellar tissue, which 

 is thereby destroyed. Soon after the pollen tube has begun to develop, 

 the generative cell forms the stalk and body cells, the latter finally giving 



