282 
PHYSIOLOGY. 
BOOK II. 
endosmose of the pollen is not a mere hypothesis, has been 
shown by Mirbel in a continuation of the beautiful memoir 
I have already so often referred to. He finds that, on the 
one hand, a great abundance of fluid is directed into the 
utricles, in which the pollen is developed, a little before the 
maturity of the latter, and that, by a dislocation of those 
utricles, the pollen loses all organic connection with the lining 
of the anther ; and that, on the other hand, these vitricles are 
dried up, lacerated, and disorganised, at the time when the 
pollen has acquired its full developement. 
The exact mode in which the pollen took effect was for a 
long time an inscrutable mystery. It was generally supposed 
that, by some subtle process, a material vivifying substance 
was conducted into the ovules through the style ; but nothing 
certain was known upon the subject until the observations of 
Amici and of Adolphe Brongniart had been published. It is 
now ascertained that, a short time after the application of 
the pollen to the stigma, each grain of the former emits one 
or more tubes of extreme tenuity, not exceeding the 1500dth 
or 2000dth of an inch in diameter, which pierce the conduct- 
ing tissue of the stigma, and find their way down to the 
region of the placenta, including within them the active mole- 
cules found in the grain. Whether or not the pollen tubes ac- 
tually reach the ovules, remains to be proved. No one has ever 
seen them in contact after the pollen tubes have arrived at the 
placenta ; for the tubes which Brown states he has traced into 
the apertures of the ovules of Orchis Morio, and Peristylus 
(Habenaria) viridis, cannot be considered an instance to the 
contrary, inasmuch as this great observer admits that the tubes 
in those plants probably do not proceed from the pollen.* 
Be this as it may, it is quite certain that it is absolutely 
necessary for the pollen to be put in communication with the 
foramen of the ovule, through the intervention of the con- 
ducting tissue of the style. In ordinary cases this is easily 
effected, in consequence of the foramen being actually in 
contact with the placenta. Where it is otherwise, nature has 
provided some curious contrivances for bringing about the 
necessary contact. In Euphorbia Lathyris the apex of the 
* See Appendix. 
