LIFE HISTORY OF PINUS 49 



instances the tube-nucleus can invariably be detected in the 

 pollen-tube. As pollen-grains containing but one cell were 

 also observed in this species on these dates, it might be 

 suggested that in the case of two cells the second prothal- 

 lial cell had persisted. The two cells, however, are exactly 

 similar to the stalk and the generative cell in their young con- 

 dition, and I see no reason for considering that they are not 

 these cells. On and after March 8 the antheridial cell of P. 

 austriaca is almost never found undivided. This date is given 

 for 1899 ; it would probably fluctuate in different years. Fig. 

 79 show N s the prophase of this division in Pinus rigida. Mi- 

 totic figures for this species have been found from April 21 to 

 May 13 of the same season. The division of the antheridial 

 cell in Pinus resinosa has been observed but once, this division 

 occurring on April u. All that can be said at present regard- 

 ing this mitosis in Pinus montana var. uncinata is that the gener- 

 ative cell and the stalk-cell are found as early as April 9. When 

 they are formed has not been determined. 



In one preparation of Pinus Strobus two of the three pollen- 

 tubes which have almost reached the prothallium are furnished 

 with sperm- and stalk-cells, while in the third only the tube- 

 nucleus is found. On the apex of the nucellus there is a 

 pollen-grain which at this late date contains one cell, the 

 antheridial cell, still undivided (fig. 73). The nucleus of this 

 pollen-grain (fig. 74) is large, plump, and to all appearances 

 perfectly normal, and it is possible, though scarcely probable, 

 that it might still have divided. That one cannot 'trace a defi- 

 nite connection between the pollen-tube containing only the 

 tube-nucleus and this pollen-grain signifies little, for those who 

 have studied the pollen-tube of Pinus know that it is the excep- 

 tion rather than the rule when a given pollen-tube can be traced 

 through the lacerated dead tissue of the upper portion of the 

 nucellus to the pollen-grain from which it proceeded. Such a 

 condition as that described is rarely met with at so late a date ; 

 but occasionally during the summer and fall pollen-grains of 

 Pinus Strobus are found in which no cell-division has taken 

 place since pollination, although in the great majority of cases 



