326 



PLANT MORPHOLOGY 



rise to two sperm mother cells (Figs. 27QC-E). From each of these a 

 motile sperm is organized. The sperms of cycads are large and multi- 

 ciliate, the cilia arising from a ])lepharoplast (Figs. 277 and 278). In 

 Microcijcas 16 to 22 sperms are formed in each pollen tube. 



After the pollen tube has penetrated the nucellar tissue and entered the 

 archegonial chamber, it ruptures and frees the sperms, which then make 

 their way toward the archegonia (Fig. 278). An entire sperm enters an 



Fig. 274. Zamia floridana. Longitudinal section of the ovule shortly after pollination, 

 X4; m, micropyle; n, nucellus; a, archegonium; g, female gametophyte; o, outer fleshy 

 layer of integument; s, middle stony layer; i, inner fleshy layer. 



egg, but its nucleus soon separates from the cytoplasm and band of cilia, 

 moving toward the egg nucleus and fusing with it. 



Embryo. The nucleus of the fertilized egg undergoes 8 to 10 simultane- 

 ous divisions (only 6 in Bowenia) without the formation of cell walls (Fig. 

 279 A). As a result, as many as over a thousand free nuclei may be pro- 

 duced. This free-nuclear stage is common to all cycads, but differences 

 now appear. In the Cycas type of embryogeny persistent cell walls are 

 formed throughout the fertilized egg. In the Dioon type cell walls 

 appear throughout the egg but soon disappear except at its base. In 

 Zamia wall formation is confined to the basal portion, not even evanes- 

 cent walls appearing in the main body of the egg (Fig. 279B). 



The cells at the base of the egg constitute the proemhryo. Even where 

 persistent cell walls appear above, the upper portion functions as a large 

 food reservoir, contributing no cells to the formation of the new plant. 



