LIFE HISTORY OF PINUS 45 



Starch-grains have been found in the large cell from an early 

 date in the development of the pollen-grain, but they are more 

 abundant after maturity is reached than at any previous time. 

 According to Coker ('02) the pollen-grains of Podocarpus con- 

 tain large starch-grains from the beginning of the first division. 

 With such variations in details as have been noted above, this 

 description of the development of the pollen-grain in Pinus 

 agrees with that given by Strasburger in 1892 and Coulter and 

 Chamberlain in 1901. 



POLLINATION. 



The Ovule at the Time of Pollination. In the vicinity of 

 Cornell University, 42^ north latitude, the pollen-grains of 

 Pirius Strobus are ready for dispersion late in May or early in 

 June, but in the other species studied pollination takes place 

 during the latter part of May. At this time the axis of the 

 female cone elongates, thus separating the ovuliferous scales 

 which now make an angle of about thirty-five degrees with the 

 rachis. After pollination the fruit scales draw together and, 

 according to Strasburger and Hillhouse ('oo), their edges are 

 consolidated by the ingrowth of papillae. The presence of two 

 ovules at the base of each scale, each ovule with its apex extend- 

 ing downwards, that is towards the base of the scale, and out- 

 wards, is too familiar a fact to need more than a passing men- 

 tion here. 



As pointed out by Hofmeister ('62) the integument is con- 

 tinued above the nucellus into two long arms which curve out- 

 ward before pollination and lead below to a wide mycropylar 

 canal. The degree of development which the ovule has obtained 

 at the time when the pollen-grains reach the nucellus is shown 

 in fig. 66. Deep within the central portion of the ovule, at its 

 chalazal end, a single cell is distinguished from the others by its 

 greater size and larger nucleus, this is the macrospore * of Hof- 

 meister ('51). The so-called "spongy "tissue of Strasburger 

 is already well differentiated when pollination takes place (figs. 

 66, plate VI, and 124, plate XII). Somewhat later the integu- 



J In 1901, I stated that, at the time of pollination, there was in the nucellus 

 an axial row of cells. I know, now, that this condition has rarely been reached 

 at so early a date, and should be noted as very exceptional rather than as normal. 



