50 MARGARET C. FERGUSON 



the stalk- and the generative cell have been formed before the 

 middle of August. 



These observations indicate that, while the division of the 

 antheridial cell takes place comparatively soon after the pollen- 

 grain has germinated in Pinus Strobus, and in some instances, 

 at least, before the winter's rest in P. austriaca, it is deferred 

 until the following spring in Pinus rigida and P. resinosa. 

 Furthermore, the time during which this cell may divide in a 

 given species may extend over several weeks, and in some cases 

 the division may never take place at all. 



The Winter Condition. A vertical section of an ovule of 

 Pinus Strobus collected on January 4 is represented in fig. 70, 

 plate VI. The spongy tissue surrounds a cavity crossed by 

 irregular strands of cytoplasm in which the free nuclei of the 

 prothallium are imbedded. In this instance the prothallium has 

 doubtless been displaced during fixation as it consists, normally, 

 at this stage, of a uniform layer of cytoplasm surrounding the 

 gametophytic vacuole and containing several nuclei. The 

 stalk- and the generative cell are enclosed within the pollen- 

 grain, and the tube-nucleus is near the apex of the irregularly 

 branched pollen-tube. This pollen-tube is shown more highly 

 magnified in fig. 83, plate VIII. At this time the pollen-tubes 

 have penetrated the nucellus almost to the point at which it joins 

 the free limb of the integument. The greatest depth to which 

 the tubes may have grown is not indicated in the illustration, but 

 this section was figured because it shows more clearly than any 

 other section in the series the cells of the pollen-grain and 

 the tube-nucleus. Other sections of the same ovule would have 

 shown pollen-tubes which had pierced to a greater depth into the 

 nucellus. The conditions of development as figured for Janu- 

 ary coincide perfectly with those which exist during the latter 

 part of October. 



THE SECOND PERIOD OF GROWTH. 



Renewed Activities in the Macrosporangium. Growth is 

 very slow during the first period of development following pol- 

 lination, but with the renewed activities of spring the ovule 

 increases rapidly in size ; the central cavity of the nucellus 



