Notes on Pollen. By Lord Avebury. 489 



Darwin, in his Forms of Flowers,* gives two interesting tables 

 showing the relative diameter of the pollen -grains from the forms 

 of the same heterostyled species, those from the short-styled form 

 being represented by 100. In the cases given " there is no excep- 

 tion to the rule that those from the anthers of the short-styled 

 form, the tubes of which have to penetrate the longer pistil of the 

 long-styled form, are larger than the grains from the other form." 



This seems very strong evidence : nevertheless he goes on to 

 say that : — " This curious relation led Delpino (as it formerly did 

 me) to believe that the larger size of the grains in the short-styled 

 flowers is connected with the greater supply of matter needed for 

 the development of their longer tubes. But the case of Limivi — in 

 which the grains of the two forms are of equal size, whilst the pistil 

 of the one is about twice as long as that of the other — made me 

 from the first feel very doubtful with respect to this view." He 

 goes on to quote certain cases (Limnanthemum, Coccocypselum, 

 Pulmonaria, Erythroxylum, Suteria), and observes that " these cases 

 seem to prove that the difference in size between the grains in the 

 two forms is not determined by the length of the pistil, down which 

 the tubes have to grow." 



He then proceeds to give the reasons which led him to doubt 

 Delpino's conclusion, and sums up as follows : — " Notwithstanding 

 these several considerations, it is difficult quite to give up the belief 

 that the pollen-grains from the longer stamens of heterostyled plants 

 have become larger in order to allow of the development of longer 

 tubes. Now if we suppose that the tubes in some heterostyled 

 species are developed wholly or almost wholly from matter con- 

 tained within the grains, while in other species from matter yielded 

 by the pistil, we can see that in the former case it would be neces- 

 sary that the grains of the two forms should differ in size relatively 

 to the length of the pistil which the tubes have to penetrate, but 

 that in the latter case it would not be necessary that the grains 

 should thus differ." His final conclusion is that the explanation 

 " must remain at present doubtful." Yet the case, even from the 

 evidence of heterostyled species, which he considered almost ex- 

 clusively, is certainly very strong. 



As regards other plants he says that : — " With plants in gener.il 

 there is no close relationship between the size of the pollen-grains 

 and the length of the pistil." This is no doubt true ; Darwin's care 

 and accuracy are well-known. But the essence of the sentence lies 

 in the word " close." The relation is indeed anything but close, 

 and there are many cases difficult to explain. Kerner also ex- 

 presses himself doubtfully on the subject. 



Nevertheless, that there is a relation seems clear, especially 

 if we compare cases belonging to the same family, and even more 

 so in the same genus. 



» Page 249. 



Octoher 16th, 1912 2 L 



