284 
PHYSIOLOGY. 
BOOK II. 
cal that it really seems to me impossible to say with which the 
discovery about to be mentioned originated : it will therefore 
be only justice if the Essays referred to are spoken of collectively, 
instead of separately. These two distinguished botanists as- 
certained that the production of tails by the grains of the 
pollen was a phenomenon connected with the action of fer- 
tilisation ; they confirmed the existence of the suture des- 
cribed by Ehrenberg ; they found that the true stigma of 
Asclepiadeae is at the lower part of the discoid head of the 
style, and so placed as to be widiin 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, occasionally, 
almost into the cavity of the ovarium ; and thus they esta- 
blished the highly important fact, that this family, which was 
thought to be one of those in which it was impossible to sup- 
pose 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 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). 
It would, therefore, seem that actual contact between the 
pollen and the stigma is indispensable in all cases. Orchi- 
deous plants, however, offer an apparent exception; for in 
them nature has, on the one hand, provided special organs, in 
the form of the stigmatic gland and the caudicle of the pollen 
masses, to assist in the act of fertilisation ; and on the other 
