156 THE LIVING CYCADS 



the origin of the free nuclear habit. This habit of free 

 nuclear division in the early stages of the female gameto- 

 phyte, having been acquired, has never been lost even 

 in the higher seed plants, which have very small gameto- 

 phytes. 



While we should not attempt, within the cycads 

 themselves, to trace an evolutionary line based upon the 

 female gametophyte, the preceding argument indicates 

 that in phylogeny the green gametophyte, cellular 

 throughout its entire existence, was the original form, 

 and that the free nuclear habit was a secondary develop- 

 ment due to the increasing size of the spore and the rapid 

 succession of nuclear divisions. The fact that the free 

 nuclei are in a homogeneous mass of protoplasm and 

 nutritive substances would account for the well-known 

 fact that the nuclei divide simultaneously until the 

 period of wall formation is initiated. 



THE EVOLUTION OF THE MALE GAMETOPHYTE 



In some respects the male gametophyte presents an 

 almost startHng uniformity throughout the entire group : 

 the pollen grain at the time of shedding consists of three 

 cells, a prothaUial cell, a generative cell, and a tube cell ; 

 the pollen-grain end of the tube remains free in the 

 pollen chamber, while the opposite end grows out into 

 the nucellus and acts as a haustorium; the generative 

 cell divides just once, forming a stalk cell and a body 

 cell; the body cell divides just once, except in Microcy- 

 cas, producing two sperms bearing numerous cilia upon 

 a coiled band. 



However, in spite of this remarkable uniformity, there 

 are small but constant differences, like the small number 



