LIFE HISTORY OF PINUS 47 



stand up, as it were, like so many fingers to catch the pollen- 

 grains ; they also serve to facilitate the entrance of the pollen- 

 tubes into the tissue of the nucellus (fig. 75, plate VII). A little 

 later this depression may become more prominent, both by the 

 slight disintegration of some of the superficial cells of the nu- 

 cellus, due to the action of the pollen-tubes, and by the incon- 

 siderable growth, after pollination, of the peripheral layer of 

 cells of the nucellar tip. The deep cup-like depression some- 

 times observed is invariably the result of abnormal disintegration. 

 The pollen-chamber in Pinus, then, consists of a space bounded 

 on the bottom by the more or less concave upper surface of the 

 nucellar tip, and arched above by the ingrowth of the free por- 

 tion of the integument. Later a resinous substance is secreted 

 which securely seals the opening by which the pollen-grains 

 entered. 



DEVELOPMENT OF THE POLLEN-TUBE. 

 THE FIRST PERIOD OF GROWTH. 



Germination of the Pollen-grain. Germination of the pollen- 

 grain follows immediately after pollination. Ovules of Pinus 

 Strobus that were fixed on June 6, 1898, had not been pollinated, 

 but on June 13 pollination had occurred and the pollen-tubes 

 had been emitted ; similar evidence could be given for the other 

 species studied, but exact data on this point are at hand for Pinus 

 rigida only. Dispersion of the pollen occurred in this species 

 in the vicinity of Wellesley College in 1902 on May 27, and in 

 material fixed two days later, May 29, the first stages of germi- 

 nation are clearly evident. It is probable that the time is not 

 longer in the other species. This confirms Strasburger's ('92) 

 statement that germination takes place in Pinus at once after 

 pollination. Hofmeister ('51) was doubtless unable to detect the 

 early stages in the germination and hence was led to the con- 

 clusion that pollination and germination were separated by 

 several weeks in the Abielinea. 



The pollen-grain increases slightly in size, the ventral or 

 concave portion of the wall becomes convex, then bulges out, 

 the exospore is ruptured, and the endospore is gradually pro- 

 longed into a tube. Immediately upon the formation of the 



