66 MARGARET C. FERGUSON 



remote from the tubes do not suffer so severely, and retain their 

 protoplasm for a much longer time. Finally all the cells 

 representing the first year's growth of the nucellar tip loose 

 their content to a greater or less degree, and their cell-walls 

 become thickened and dead. During the rapid growth of the 

 pollen-tubes through that portion of the nucellar cap which 

 develops the second season, the effect of the tubes on the sur- 

 rounding tissue is less marked, though here, too, the cells with 

 which they come into contact are crushed and destroyed (fig. 

 73, plate VII). I have made no physiological investigations 

 regarding the action of these tubes on the tissue of the nucellus, 

 but, judging from the disappearance of the starch in the cells 

 just in advance of the tubes and the gradual disintegration of 

 those cells, it seems very probable that the destruction of tissue 

 is not due to mechanical reasons alone, but to the action of some 

 ferment or digestive substance as well. Various views have 

 been expressed concerning the action of the pollen-tube and the 

 directive agent in its growth by Molisch ('93), Miyoshi ('94), 

 Lidforss ('99) and others, but we are still far from a clear under- 

 standing as to the controlling factor in the movement. The 

 pollen-tube cannot be guided to the egg in Pinus by any peculiar 

 attraction existing between the sexual cells, for it grows with 

 normal rapidity when no sperm-cells are formed, and also when 

 the archegonia are in a state of disintegration. 



SUMMARY. 



Upon the germination of the microspore, three divisions fol- 

 low in rapid succession giving rise to the pollen-grain. At the 

 close of the prophase of each division the karyokinetic figure 

 is pointed at its lower extremity and very broad at the extremity 

 in contact with the dorsal side of the young pollen-grain. The 

 inner, incomplete, thick wall formed in the development of the 

 microspore persists as a part of the mature pollen-grain. It 

 probably serves as a strengthening layer, particularly at those 

 points at which the wall has been weakened by the expansion of 

 the exospore. When the telophase of the second division is 

 reached the first prothallial cell has become flattened against the 

 convex side of the spore-wall, its cytoplasm has been withdrawn, 



