LIFE HISTORY OF PINUS 53 



dant than that of the tube-nucleus, is still delicate and often 

 shows a weak reaction to nuclear stains. The stalk-nucleus 

 has a very decided individuality which it maintains throughout 

 its entire history. It bears a strong resemblance from the first 

 to the nuclei of the nucellar tissue ; rarely, if ever, contains a 

 true nucleolus ; and its close-meshed reticulum is conspicuous 

 for its comparatively large net-knots or karyosomes. 



Division of the Generative Nucleus. — Comparatively few 

 students have occupied themselves with the growth of the pol- 

 len-tube in the Abietincce, and no one, in so far as I have been 

 able to determine, has described the cytological features attend- 

 ing the formation of the sperm-nuclei in this group. 



Dixon ('94) describes this division in Pinus sylvestris as tak- 

 ing place about a month before fertilization^ while the genera- 

 tive cell is still ivithin the pollen-grain ; and Coulter ('97) 

 states, as already mentioned, that in his study of Pinus Laricio 

 he has been able to confirm Dixon's observations in the minutest 

 detail. At this time, as pointed out by Dixon, the nuclear and 

 cytological phenomena are very greatly obscured by the pres- 

 ence in the pollen-tube of large quantities of starch (fig. 91). 

 The starch, which resists the microtome knife and is therefore 

 easily displaced by it, not infrequently falls out and carries 

 away with it the free cells of the pollen-tube. The dead, deeply 

 staining tissue of the nucellus, representing that portion of the 

 nucellar cap which was penetrated by the pollen-tubes during 

 the previous season, and in which the generative nucleus divides 

 (fig. 72, plate VI) is also very troublesome. Furthermore the 

 dense cytoplasm of the generative cell has a great affinity for 

 stains, so that when the archegonia and other portions of the 

 ovule are well stained, this cell often appears merely as a deeply 

 stained mass showing no differentiation of parts. Considering 

 the fact that I was led not only to expect this division to 

 take place within the pollen-grain but to search for it 

 some weeks earlier than it actually occurs in the species 

 of pines studied, together with the difficulties of staining, it is 

 not surprising that seven hundred slides of serial sections were 

 made, which means that more than two thousand pollen-tubes 

 were studied, before any definite clue was obtained as to the 



