LIFE HISTORY OF PINUS 6'J 



andthe nucleus has lost all signs of its former structure remain- 

 ing as a much flattened, deeply staining mass. At the close of 

 the third division, the second prothallial cell has suffered a simi- 

 lar fate. Both prothallial cells are furnished with cellulose-walls. 



In the mature pollen-grain the prothallial cells are usually 

 represented by two broken, dark lines along the dorsal side of 

 the pollen-grain, but all vestiges of the first cell may have dis- 

 appeared. The antheridial cell projects from the convex side 

 of the spore at its middle point, and the tube-nucleus is always 

 directly below but in contact with the antheridial cell. Starch 

 is found in the pollen-grain at maturity and during its develop- 

 ment. 



Pollination takes place between 42° and 43° north latitude 

 during the latter part of May or the first ten days in June. At 

 this time the macrospore-mother-cell is distinctly visible in the 

 center of the ovule, but slightly nearer its basal end. 



In the young ovule the free portion of the integument, above 

 the tip of the nucellus, consists in cross-section of three layers 

 of cells. After pollination the arms of the integument become 

 erect, thus bringing the pollen-grains into the wide micropylar 

 canal. Then the inner layer of cells just above the pollen- 

 grains elongates rapidly, extending inwards and meeting at the 

 center. The pollen-grains having thus been made secure, the 

 elongated cells become divided into many small cells. It is 

 felt that the pit in the apex of the ovule in Piniis has been ex- 

 aggerated. There is rarely more than a slight concavity before 

 pollination. Through the action of the pollen-tubes it may be 

 somewhat deepened, but in normal conditions it does not become 

 " cup-like." 



Two days after pollination, in Pinus rigida, the pollen-tubes 

 have been emitted. In the other species germination has been 

 shown to take place in less than a week after pollination, but 

 more exact data have not been obtained for these species. As 

 soon as the pollen-grain has germinated, the tube-nucleus severs 

 its connection with the antheridial cell and moves into the elon- 

 gating tube. 



The division of the antheridial cell takes place in Pinus 

 Strobus during the first week in August. It sometimes divides 



