LIFE HISTORY OF PINUS 69 



equality in the size of these nuclei is very early apparent, and 

 becomes more pronounced as they reach maturity. The sperm- 

 nuclei soon come to lie together in the upper part of their cyto- 

 plasm and quickly attain their full size, the larger one being 

 invariably in advance. The nuclear reticulum, at first delicate, 

 soon becomes very dense, but there is no evidence of the pres- 

 ence in these nuclei of a special metaplasmic substance. 



During the division of the generative nucleus the ovule in- 

 creases much in size, and the nucellar cap becomes several 

 times deeper than during the first season, thus carrying the 

 upper portion of the nucellus with its pollen-tubes far above the 

 endosperm. 



At the time when the sperm-nuclei come into contact, or 

 nearly so, the pollen-tube has penetrated little, if at all, beyond 

 the nucellar tissue of the first year's growth. Now, however, 

 it again begins to elongate, and its downward course through 

 the new nucellar tissue is extremely rapid. The destruction of 

 the nucellar tissue through which the pollen-tubes travel, ap- 

 parently results not only from mechanical disturbances, but from 

 the entire dissolution of some of the cells through the action of 

 a ferment. 



When just above the egg, the apex of the pollen-tube is filled 

 with cytoplasm. The tube-nucleus lies in the upper part of the 

 cytoplasm, and near it is seen the stalk-cell still in contact with 

 the lower portion of the cytoplasm which surrounds the sperm- 

 nuclei. 



The existence of the diffused centrosome is suggested in con- 

 nection with the division of the generative nucleus, and there is 

 a possibility that, in the prominent cytoplasmic figure from 

 which the spindle takes its origin, we may have represented, in 

 its vestigial state, the cilia-forming body found In the lower 

 Gymnosperms. 



