CYCADALES 



127 



THE MALE GAMETOPHYTE 



The microspore is the first cell of the male gametophyte. It has a 

 very definite polarity and two very definite spore coats. The exine 

 is thick at the bottom, moderate on the sides, and thin at the top; 

 while the intine is thin at the bottom and top and very thick on the 

 sides (figs. 134-37)- 



The microspore, in all the cycads, begins to germinate while still 

 contained in the microsporangium. At the first mitosis, a prothallial 

 cell is cut off. This cell does not degenerate, as in most gymnosperms, 



Outer fleshy layer- - 

 Stony layer- 

 Inner fleshy layer- 

 [ndosperm- 



'ucellus 



/33 



Fig. 132. — Dioon edule: megasporophyll with two ovules; one-half natural size. 

 Fig. 133. — Dioon edule: section of ovule at the stage shown in fig. 132; X2. Both figs. 

 132 and 133 from Ch.amberlain, The Living Cycads"° (University of Chicago Press). 



but becomes very active, although it never divides. The other cell 

 divides, forming a generative cell and a tube cell. At this three- 

 celled stage the microspore is shed from the sporangium. 



There have been many reports of insect pollination, but in a rather 

 extensive field study in which all of the genera have been examined, 

 most of them at the time of pollination, nothing has been observed 

 which would indicate anything but the wind pollination so charac- 

 teristic of the whole group of gymnosperms. The pollen is light and 

 dry, and easily blown by the wind. The entire mass of pollen in a 

 microsporangium, for a while, hangs together in one mass, being 

 held by a kind of membrane formed by the disorganized wall and 

 tapetal cells. When this breaks, the dry pollen is shed. As soon as 

 the microsporophylls begin to crack apart, insects arrive and can be 



