PINUS 185 



10. The ovules are megasporangia surrounded by a 

 protecting integument. 



1 1 . There is some evidence for considering the ovulif- 

 erous scale and the bract that subtends it as a 

 megasporophyll, or carpel. On the basis of this 

 interpretation the bract would be homologous to the 

 ligule in Isoetes or Selaginella. But other facts 

 argue against this theory, and lead to different 

 interpretations, so that the exact homology of the 

 organ is in doubt. Possibly it represents two mega- 

 sporophylls or carpels. If so, we must interpret the 

 carpellate cone, not as a flower, like the staminate 

 cone, but as inflorescence, or cluster of flowers, each 

 scale representing a flower. 



/. The Mature Male Gametophyte: 



During the first spring pollination takes place, as 

 described in the text-book, and the growth of the 

 pollen-tube begins. Its growth is very slow, however, 

 until the following spring, when the growth becomes 

 more vigorous. The tube-nucleus passes to the tip of 

 the pollen-tube, which penetrates the tissues of the 

 nucellus (/, 6, p. 156), digesting a channel for itself as it 

 grows, usually branching, and feeding on the digested 

 tissue. The generative cell divides into a body-cell 

 and a stalk-cell, and the nucleus of the body-cell 

 again divides into two sperm-nuclei. 



K. The Female Gametophyte: 



Near the time of pollination the megaspore consists of 

 one uninucleate cell (the one-celled stage of the embryo- 

 sac). By repeated nuclear-divisions the nucleus of the 

 megaspore gives rise to a large number of nuclei, which 

 at first lie free in the surrounding cytoplasm; but later 

 each of these nuclei organizes about itself a cell, 

 surrounded by cell-walls. The tissue thus formed 



