230 REPRODUCTION 



The fertilized egg now begins a series of divisions leading to 

 the formation of the embryo, while the primary endosperm 

 nucleus gives rise to an endosperm or food-storage tissue of 

 greater or less magnitude, sometimes constituting the bulk of 

 the seed, as in castor bean, or dividing the seed about equally 

 with the embryo, as in Indian corn, or holding itself in abeyance 

 and only assisting the embryo to take up the food about as 

 fast as it comes for storage, as in the Lima bean and other 

 Leguminosse. 



The division of the endosperm nucleus usually precedes that 

 of the egg, as illustrated in Fig, 131, L, where the embryo has 

 reached the two-celled stage, while five endosperm nuclei are 

 in process of division. 



The Triple-fusion Nucleus. — The triple-fusion nucleus 

 formed by the fusion of the two polar nuclei to form the primary 

 endosperm nucleus, and the union of this with one of the male nu- 

 clei, has still no assured interpretation. One of the polar nuclei 

 is sister to the egg-cell nucleus, and it might be expected that the 

 fusion of a male nucleus with this would produce a second em- 

 bryo. It has been suggested that the presence of the second 

 polar nucleus is a disturbing element in this; but in some instances 

 only one polar nucleus fuses with the male nucleus, and the 

 result is the same as when both polar nuclei take part. Again 

 it may be that the fusion of these nuclei is not perfect and not 

 of the same nature as the fusion of the sperm and egg cell. This, • 

 if true, would class the endosperm with the gametophyte gener- 

 ation (see page 218), where the general judgment still 

 places it. 



That the male nucleus may affect the character of the endo- 

 sperm is most clearly shown in Indian corn. The wrinkling 

 of sweet corn is due to the fact that sugar is contained in solution 

 in the cell sap, instead of starch that fills the cells of dent and 

 flint corn, and therefore on drying sweet corn shrinks while field 

 corn remains plump. Now when sugar corn is fertilized by field 

 corn the grains turn out plump and starchy instead of sugary, 

 the second male nucleus from the microspore having trans- 



