I04 ' THE LIVING CYCADS 



stages strictly comparable with those leading up to the 

 formation of sperms in animals. At a very early stage 

 in the development of the microsporangium one or more 

 cells just beneath the epidermis become somewhat 

 larger than their neighbors and richer in protoplasm. 

 They divide several times, all dividing at once, so that 

 each division doubles the number of cells. After a while 

 these divisions cease, the cells separate from each other 

 and become spherical, at which stage they are called 

 ''microspore mother-cells," because each one, by two 

 very peculiar divisions, gives rise to four microspores. 

 It is during these two divisions that the number of 

 chromosomes is reduced, and the plant returns to the 

 original gamete-bearing, or gametophyte, generation.' 

 The counterpart is the sporophyte generation, whi{:h 

 begins with the fertilized egg and constitutes all we can 

 see of any flowering plant, except by means of the micro- 

 scope. These two generations alternate, the sporophyte 

 producing the gametophyte, and the gametophyte in 

 turn producing the sporophyte. Such an alternation is 

 a necessary consequence of fertilization in plants and, we 

 believe, in animals also. 



The microspore then is the first cell of the male 

 gametophyte. In the megasporangium (ovule) only 

 four spores are produced, and only one of these func- 

 tions, but in the microsporangium the number of spores 

 is very large, even larger than in any living ferns. In 

 Dioon edule the number of spores may reach 30,000, in 

 Encephalartos villosus 26,000, in Ceratozamia mexicana 

 8,000, and in Zamia floridana ^00. 



The microspore consists of a single cell with a wall 

 differentiated into two distinct layers, the outer called 



