CHAP. VIII. 
FERTILISATION. 
341 
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, at times so nearly 
identical that it seems to me impossible to say with which 
the discovery about to be mentioned originated : it will 
therefore be only j ustice if the Essays referred to are spoken 
of collectively, instead of separately. These two distinguished 
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 de- 
scribed by Ehrenberg; they found that the true stigma of 
Asclepiadaceae 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, into the tissue of which they buried themselves so 
perceptibly, that they were enabled to trace them, occasion- 
ally, 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 be- 
tween 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 differ- 
ences 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). 
z 3 
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