No. 472] 



POLLEN GRAIN VARLiTION 



265 



ture of the pollen grain in gymnosperms is at the present time in a 

 state of stable equilibrium or in a process of retrogressive modifi- 

 cation. Coker's Figures 6 and 7 will be further noticed in con- 



PlCEA EXCELSA 



In presenting my own observations I have considered it desir- 

 able to illustrate rather fully the different variations found in the 

 pollen grains of Picea cxcelsa L., and these illustrations show a 

 surprising range in the variation in internal structure of the pollen. 

 One fact that seemed especially significant was that so few pollen 

 grains could be found showing satisfactorily what has been called 

 the "normal" structure, that is showing two disintegrating pro- 

 thallial cells. For the larger number of pollen grains showed 

 only one such disintegrating cell without a trace of a second, even 

 as a split in the wall of the pollen grain. In order to get at the 

 proportion of those which showed only one disintegrating pro- 

 thallial cell, 466 pollen grains were counted. Only those were 

 counted which lay in the position most favorable to the iir(c»arv 

 observation, that is, the side view as shown in Figs. 1(1 I'l. 1 . 

 No pollen grain was counted unless its internal stnu turt> x ciikm! 

 reasonably clear. Of the total of 466 grains counted, ;!10 or <ir..,V , 

 showed only one disintegrating prothallial cell with no tract- of 

 another even as a split in the wall; 73 or 15. 7S .sii(»\ved more or 

 less plainly two disintegrating prothallial cells; 18 or :^.S' , showed 

 no disintegrating cells; 65 or 13.9% were doubtful as to whether 

 they had one or two disintegrating cells. Even if all the (h)iil)tful 



showed only one prothalHal cell I still nii>rc than twice a. ,-reat 

 as the "normal." Of course it may he said that in tlio.e uniins 

 in which only one prothallial cell showed, the <.ne fir^t formed had 

 completely di>a[)i)eare(l. Hut if it is not to he seen, what evidence 

 is there that it was ever fornu'd r Two reaM)ns may i..'rha[>s he 

 given for believin- in its formation and disappearance. First, 

 in many cases in which two prothallial ccll> mav l.c n-ouiiize.l 

 the one first formed is barelv recognizal>le a. a mere dit in the wall 

 of the pollen grain, and it is probable that in >om.' < a.e.> the <rll 



