224 ACTION OF POLLEN-TUBES. [BOOK ir. 



action connected with fertilisation, and he thought that they 

 were of a different nature from the pollen-tubes of other 

 plants : he particularly observed in Asclepias syriaca that the 

 tails become exceedingly long, and hang down. 



In 1831, the subject was resumed by Brown in this country, 

 and by Adolphe Brongniart in France. These two distin- 

 guished botanists ascertained that the production of tails by 

 the grains of the pollen was a phenomenon connected with 

 the action of fertilisation; they confirmed the existence of 

 the suture described by Ehrenberg; they found that the 

 true stigma of Asclepiads is at the lower part of the discoid 

 head of the style, and so placed as to be within reach of the 

 suture through which the pollen-tubes or tails are emitted ; 

 they remarked that the latter insinuated themselves below 

 the head of the style, and followed its surface until they 

 reached the stigma, in the tissue of which they buried them- 

 selves so perceptibly, that they were enabled to trace them, 

 occasionally, almost into the cavity of the ovarium ; and thus 

 they established the highly important fact, that this family, 

 which was thought to be one of those in which it was 

 impossible to suppose that fertilisation takes place by actual 

 contact between the pollen and the stigma, offers the most 

 beautiful of all examples of the exactness of the theory, that 

 it is at least owing to the projection of pollen-tubes into the 

 substance of the stigma. In the more essential parts these 

 two observers are agreed : they, however, differ in some of 

 the details, as, for instance, in the texture of the part of the 

 style which I have here called stigma, and into which the 

 pollen-tubes are introduced. Brongniart both describes and 

 figures it as much more lax than the other tissue ; while, on 

 the other hand, Brown declares that he has in no case been 

 able to observe " the slightest appearance of secretion, or any 

 differences whatever in texture between that part and the 

 general surface of the stigma " (meaning what I have described 

 as the discoid head of the style) . 



I have remarked that, in Morrenia odorata, an Asclepiad, 

 the emission of tubes takes place to such an extent as to give 

 the head of the stigma altogether the appearance of a mass 

 of tow. (See Botanical Register, 1838, Misc. No. 129.) 



