212 Nichols, A morphological study of Juniperus communis var. depressa. 



Pollination. 



At the time of pollination the micropyle of the ovule is wide 

 open (fig. 1), and "from each orifice there is exuded a minute 

 globule of clear, shining liquid which rests like an irridescent bubble 

 on the tip, and serves to catch the pollen and conduct it to the 

 nucellus within" (Jack 1893). Here the microspores usually lodge 

 in the irregulär, saucer-shaped depression produced by the partial 

 breaking down of the superficial cells of the nucellus, and within 

 a few days the micropylar canal is almost completely closed by 

 the growth of the inner layer of cells of the integument at the 

 region near the tip of the ovule (fig. 2). Noren (1907) is of the 

 opinion that this closure is incited by the entrance of the pollen, 

 and he finds that in cases where extraneous pollen is present, but 

 none of Juniperus, the micropyle remains open, an Observation 

 which the writer can confirm. 



The male gametophyte. 



Development of the pollen tube. — As first noted by 

 Hofmeister (1851), a little over twelve months elapses between 

 pollination and fertilization. A similar prolonged period of growth 

 is characteristic of the male gametophyte in Pinus and Cephalo- 

 taxus, but in the majority of Gymnosperms, among them Juniperus 

 virginiana, only a few weeks or months intervene between polli- 

 nation and fertilization. Sludsky (1905) maintains that in J. 

 communis also the development of the male gametophyte is com- 

 pleted in a Single season, but the investigations of Noren (1907) 

 and of the writer prove conclusively that this is not the case. 



In the material studied, no microspores were found which 

 showed the primary nucleus in the act of dividing. This division 

 takes place within a week after the pollen reaches the nucellus 

 and results in two nuclei slightly different in size from one another, 

 the smaller of which — the generative nucleus — immediately 

 becomes invested with a dense layer of cytoplasm and is separated 

 from the protoplasm of the cell by a thin plasma membrane or 

 'Hautschicht'. Soon after this division the exine of the microspore 

 is ruptured, usually on the side toward the nucellus, and the intine 

 is pressed outward (figs. 40, 41). The growth of the pollen tube 

 proceeds rather slowly throughout the summer months and then 

 ceases altogether until the following spring. During this period 

 of activity the tube presses into the tissue in the upper part of 

 the nucellus, disorganizing the cells with which it comes in contact, 

 and branches somewhat, thus anchoring itself firmly (figs. 42, 43). 

 Frequently the tube wanders across the top of the nucellus before 

 penetrating it, and in many cases, after forcing its way into the 

 nucellar tissue, the tube turns sharply and grows for some distance 

 in a horizontal direction. During the first season's growth the 

 vegetative nucleus occupies a position a short distance from the 

 growing end of the tube, while the generative cell lies close against 



