68 MARGARET C. FERGUSON 



during the summer and fall in P. aush'iaca^ but, as a rule, the 

 division takes place in this species very early in March. This 

 mitosis has been observed in P. resinosa during the second 

 week of April, and in P. rigida from the middle of April to 

 the middle of May. It is evident that this cell does not always 

 divide at a definite and fixed time, but that in a given species 

 the time during which it may divide extends over a considerable 

 period. 



During the first season the pollen-tube grows very slowly, 

 and it may be broad and irregular in outline or it may branch 

 freely. 



Shortly before fertilization the generative cell, followed by 

 the stalk-cell, moves into the pollen-tube. The stalk-cell soon 

 passes the generative cell and takes up a position near the tube- 

 nucleus. These changes and those immediately following are 

 frequently much obscured by the presence in the pollen-tube of 

 large quantities of starch. 



When the macrosporangium enters upon the winter's rest, the 

 pollen-tubes have penetrated nearly to the line at which the in- 

 tegument becomes free from the nucellus and the tube-nucleus 

 maintains its position in the apex of the pollen-tube. 



The generative cell is never limited by a well-defined cell- 

 wall, and consists at the time of its division of an irregular pro- 

 toplasmic body in the upper part of which the nucleus lies. 



In the division of the generative nucleus the spindle is extra- 

 uuclear arid unipolar in origin, a unique and heretofore unob- 

 served method of division. 



The formation of the spindle indicates that the cytoplasmic 

 network and the nuclear reticulum have essentially the same 

 structure, and the spindle-fibers are apparently formed by a 

 transformation of both. The nuclear membrane persists along 

 the upper part of the nucleus until the earl}^ stages in the forma- 

 tion of the daughter-nuclei. This division takes place a little 

 more than a year after pollination and from a week to ten days 

 before fertilization, nearly thirteen months elapsing between pol- 

 lination and fertilization. 



Two sperm-cells are never formed, but the sperm-nuclei 

 remain surrounded by a common mass of cytoplasm. An in- 



