400 



GENERAL MORPHOLOGY OF PLANTS 



nucellus which is filled with a fluid and lies next the egg cases in 

 the female prothallium (endosperm). The central cell (or stalk 



Fig. 383- 



A, section of ovule of zamia, partly diagrammatic, showing germinating pollen tubes 

 entering the nucellar cap. Af, micropyle; O, outer portion of ovule (exocarp); 7, inner 

 stony portion of ovule (endocarp) ; PC, pollen chamber; N, nucellar cap; P, endosperm (pro- 

 thallium); J,archegonium (pollen grains in pollen chamber are germinating, the pollen tubes 

 growing in the tissue of the nucellar cap). B, same a little later, showing basal end of the 

 pollen tubes bending downward as the sperm cells in that end are developing. 



cell sometimes called) has divided, as in pines, into a sterile cell 

 and a generative cell (body cell) which now divides into the two 

 sperm cells,* which are oval in form, and each has a spiral band 

 of numerous cilia around the smaller end (fig. 387). Some of 



Fig. 384- 



Zamia. A, mature pollen grain showing within, the tube cell at the right, the central 

 cell in the middle, the prothallial cell at the left; B, beginning of germination of pollen 

 grain; C, farther stage in the germination of pollen grain, the tube nucleus of cell moving into 

 the tube. (After Webber.) 



these sperms swim into the egg cases, one unites with an egg 

 nucleus, and the embryo is then formed, thus making a seed. 



* Two sperm cells are formed in most cycads as in other seed plants, but 

 in Microcycas there are eight generative cells, each of which divides to form 

 two sperms, making sixteen in all for each small spore. 



