POLLEN TUBES 115 



characteristic of the Asclepiad flower, was given. Twenty-two 

 years later (in 1831) he again returned to the Asclepiads and 

 described and discussed the mode of pollination and fertilisation 

 in this Order and also in that of the Orchids, 



It was characteristic of Brown that he clung with great 

 tenacity to any problem that had once excited his interest. 

 He made himself fully acquainted with the work of his contem- 

 poraries and predecessors, and at the same time he constantly 

 attacked it by reiterated first-hand investigations, testing hypo- 

 theses and theories by the light of direct observation. He was 

 very cautious, and thus, although he traced the pollen tubes from 

 the pollen grain into the ovary and into the micropyle (foramen) 

 of the ovule, he still leaves it an open question whether, in all 

 cases, anything of a material nature passes from the pollen to 

 the interior of the ovule, which may thus be held responsible 

 for the formation of an embryo. 



He cites the observations of Amici and of Du Petit Thouars, 

 and then states he does not feel he is as far advanced as these 

 observers. But in the succeeding pages he traces the tube, of 

 which he says, " the production is a vital action excited in the 

 grain by the application of an external stimulus." We see here 

 a clear perception of the facts of germination and of the opera- 

 tion of what we now call chemiotaxis, for he goes on to add 

 "The appropriate and most powerful stimulus to this action is no 

 doubt contact, at the proper period, with the secretion or surface 

 of the stigma of the same species. Many facts, however, and 

 among others the existence of hybrid plants, prove that this is 

 not the only stimulus capable of producing the effect ; and in 

 Orchideae I have found that the action in the pollen of one 

 species may be excited by the stigma of another belonging to a 

 very different tribe." It is hard to believe that these lines were 

 written so long as 80 years ago. Brown goes on to describe the 

 change that follows impregnation, and the gradual appearance 

 of the embryo. And we must remember that all these observa- 

 tions were made by one who relied almost exclusively on the 

 simple microscope and the simplest I had almost said barbaric 

 technique. 



He expresses himself in very reserved terms as to the nature 



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