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 utricles 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 w^as 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 ISOOdth 

 or 2000dth of an inch in diameter, which piei'ce 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 

 contrai'y, 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 

 pi'ovided some curious contrivances for bringing about the 

 necessary contact. In Euphorbia Lathyris the apex of the 



* See Appendix. 



