LIFE HISTORY OF PINUS 4I 



parts by prominent cross walls which are continuous with the 

 inner portion of the mother-wall. The microspores are then 

 developed each in its own particular chamber of the mother- 

 cell. 



A double wall is quickly developed about each spore and the 

 air-sacs become apparent while the spores are still within the 

 mother-wall. They arise by the separation of the extine from 

 the intine at two definite points on opposite sides of the spore. 

 By the growth of the spore, and more especially by the expan- 

 sion of the air-sacs, the spore-mother-wall is ruptured and the 

 spores set free. 



Growth ensues, the extine becomes irregularly thickened on 

 its inner surface except at the concave side of the spore, and a 

 broad partial wall is laid down just within the intine and along 

 the back and sides of the microspore. During the growth of 

 this cell its nucleus maintains a position at the central point of 

 its dorsal side. Before the germination of the microspore it 

 attains to the full size of the mature pollen-grain. 



CHAPTER II. 

 The Male Gametophyte. 



the development of the pollen-grain. 



Formation of the ProthaUial Cells. — So much confusion 

 has arisen in the application of terms used to designate the 

 various cells of the male gametophyte in Gymnosperms that it 

 is desirable, if not almost necessary, that one should define at the 

 outset the nomenclature adopted. Throughout this paper, the 

 first two cells cut off from the larger cell are known respectively 

 as the first and second prothallial cells, and the third small cell 

 formed represents the antheridial or third prothallial cell. The 

 large cell, so long as it continues to divide, is designated as the 

 apical cell, but after division ceases in this cell it is referred to 

 as the tube-cell and its nucleus constitutes the tube-nucleus. 

 The antheridial cell divides to form the stalk-cell and the gen- 

 erative cell, the latter giving rise to the binucleated sperm-cell. 



Proc. Wash. Acad. Sci., July, 1904. 



