LIFE HISTORY OF PINUS 5^ 



becomes greatly enlarged and is lined with the growing endo- 

 sperm. The cells of the nucellar cap which are penetrated by 

 the pollen-tubes during the previous season do not again become 

 active, but remain as deeply staining, thick-walled, dead cells. 

 The cells just beneath them, however, multiply rapidly, and 

 become literally packed with large starch-grains. A few of 

 the cells from this portion of the nucellar cap represented in 

 fig. 73, plate VII, are shown more highly magnified in fig. 89, 

 plate VIII. By the growth and increase of these cells, the dead 

 top of the nucellus with its pollen-tubes is lifted far above the 

 developing endosperm, so that the pollen-tubes, once so near 

 their goal, are now removed from it by a considerable distance 

 (figs. 70-72, plate VI). 



Renewed Activities in the Male Gametofhyte. — During the 

 rapid development of the ovule in the spring, the pollen-tube 

 increases little, if at all, in length, renewed activities in the male 

 gametophyte being first indicated by a further development of 

 the cells within the pollen-grain. 



The stalk-cell increases in size and its cytoplasm assumes a 

 vacuolate character. The growth of the generative cell is still 

 more marked, and its cytoplasm on the contrary becomes dense 

 and deeply staining. (Compare fig. 83, January 4, with fig. 

 84, May 3, plate VIII.) In Pinus syhestris, as studied by Dixon 

 ('94) and confirmed by Coulter ('97) in Pimis Laricio, the gen- 

 erative cell divides while it is within the pollen-grain. In the 

 species of pines which I have investigated, this division does 

 not occur until the generative and the stalk-cell have entered 

 the pollen-tube and the stalk-cell has passed below the gen- 

 erative cell. As the generative cell increases in size it stretches 

 out towards and into the neck of the pollen-tube, drawing after 

 it the stalk-cell, or possibly being forced out by that cell, the 

 two passing into the tube together. 



Dixon states that only the naked nucleus of the stalk-cell 

 enters the polfen-tube, and in so far as I am aware, no writer 

 has described the entrance of the entire stalk-cell into the pollen- 

 tube in Pinus. The material which I have studied shows con- 

 clusively that the nucleus does not '* slip out " of its cytoplasm 

 (figs. 83-86). The entire cell can be identified in the tube and 



