STRASBURGER ON FOREIGN POLLINATION. 



373 



from them. It is not easy, indeed, to prevent a stigma from being 

 fertilised by its own pollen, nor to distinguish this from pollen 

 derived from a foreign source. But speaking generally, Strasburger 

 states that species belonging to different genera only seldom form 

 pollen-tubes on one another, but that species of the same genus 

 mostly do. From this he infers that there can be no question of 

 special protection here, since it would be as much required for allied 

 species as for allied genera. 



Summing up the whole of the evidence on this head, our author 

 thinks it follows that in wind-fertilised phanerogams no special 

 protection exists to prevent the entrance of pollen-tubes, and the 

 same may probably be said of Gymnosperms, though he confesses 

 he has made no experiments upon them. 



It would seem then, from what is here recorded, that there are no 

 protective adaptations to prevent the germination of foreign pollen 

 on the stigma and the entrance of its tube into the style, and even 

 the ovary, and that the germination and entrance of foreign pollen 

 is possible within the widest limits, even from Monocotyledons to 

 Dicotyledons, and the reverse. That foreign pollen-tubes do not 

 often come to the ovary, and seldomer find their way between the 

 ovules, is connected with the circumstance that here the disadvan- 

 tageous influences to which they are exposed in the foreign environ- 

 ment are accumulated. In no case will a pollen-tube find in a foreign 

 plant quite the same conditions as in its own, the specific distinctions 

 of the two plants pre-supposing the specific variety of their protoplasm. 

 Moreover, where foreign pollen reaches a stigma, there is little or no 

 disadvantage to the plant, as the normal development of the proper 

 pollen is not hindered by the presence of the foreign, even though 

 the latter puts forth pollen-tubes which traverse the style and reach 

 the ovary. 



The wide range within which heterogeneous pollination is 

 possible, and the absence of protective contrivances against it, would, 

 at first sight, lead us to expect a much more frequent occurrence of 

 spontaneously-produced hybrids than experience shows is the case. 

 But, as Strasburger shows, the very processes which bring about 

 pollination limit the production of such hybrids to an extremely 

 small number. Many circumstances conspire to bring legitimate 

 pollen to a stigma, whether by the agency of insects, the wind, or 

 otherwise, and it will usually happen that a flower will be legitimately 

 pollinated several times, so that if by any chance foreign pollen be 

 brought, it will be debarred from operating by the super-abundance 

 and prepotency of the proper pollen. As a proof, however, that 

 spontaneously produced hybrids do occur, Strasburger refers to the 



Dec. 1886. 



