CHAP. VIII. 
FERTILISATION. 
339 
endosmose of the pollen is not a mere hypothesis, has been 
shown by Mirbel in a continuation of the memoir 1 have 
already so often referred to. He finds that, on the one hand, 
a ffreat abundance of fluid is directed into the utricles in 
O 
which the pollen is developed, a little before the maturity 
of the latter, while, 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 utricles are dried 
up, lacerated, and disorganised, at the time when the pollen 
has acquired its full developement. 
Morren has made some statistical observations upon the 
sexual organs of Cereus grandiflorus. He found that in each 
flower of this plant there are about 500 anthers, 24 stigmata, 
and 30,000 ovules. He estimates each anther to contain 
500 grains of pollen ; the whole number in each flower being 
250,000 ; so that not more than an eighth of the whole 
number of pollen grains can be supposed to be effective. 
The distance from the stigma to the ovules he computes at 
1150 times the diameter of the pollen grain. 
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 1500th 
or 2000th of an inch in diameter, which pierce the con- 
ducting tissue of the stigma, and find their way down to the 
region of the placenta, including within them the molecular 
matter found in the grain. These pollen tubes actually 
reach the ovules. Brown states he has traced them into the 
apertures of those of Orchis Morio, and Peristylus (Ha- 
benaria) viridis, although this great observer adds 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 
z 2 
