:^o. 472] POLLEN GRAIN V ABLATION 



279 



o has come about by gradual modification, and 

 tioned show that the "normal" structure is not 

 so firmly fixed but that a small proportion of the grains produced 

 reverts to a condition which was a stage in the gradual develop- 

 ment. If this interpretation of the variations is rejected, then they 

 must be looked upon as mutations, for whose cause we have noth- 

 ing at all to suggest. 



It is worth noting that among the gymnosperms the reduction 

 of one structure in the pollen grain is not necessarily accompanied 

 by the reduction of other structures in the same pollen grains. For 

 example in Cupressus (Coker, : 04) no sterile prothallial cell is cut 

 ofl", and yet many sperm cells are formed instead of only two (Juel, 

 :04). On the other hand, Picea, which may have as many as 

 three sterile prothallial cells not only has the sperms reduced to 

 two nuclei in one cell but even one of those nuclei has undergone 

 a further reduction in size and only one, the larger, is functional 

 (Miyake, : 03). In the reduction of the sperm cells therefore, 

 some of the gymnosperms have been modified to even a greater 

 extent than the angiosperms. The degree of reduction seems to 

 be closely associated with "use and disuse" of the parts concerned. 

 Reduction of one of the sperm cells takes place only in cases where 

 both could not function in fertilizing the egg. In the angiosperms 

 the second sperm cell functions by uniting with the polar nuclei 

 to form the en.lospcrm, and this functioning whether a true fertil- 

 ization or not, luis been sufficient to prevent the reduction and 

 possible suppression of one of the sperm cells. 



The observations of Miyake (:03) that in Ahles balsamea the 

 second sperm nucleus may divide, or attempt to divide before it 

 disintegrates in the upper part of the egg, and especially the obser- 

 vation that in one case this second sperm nucleus united \^•ith one 

 of the second segmentation nuclei of the fertiliz<"(l fu<;. is >ui:iies- 

 tive as to the possible way in which "doul)le fertilization" may 



all necessary that either of the polar nnclci be the homologue of 

 an egg as Bonnier (M).')) believes, in order to explain the union 



