218 EMISSION OF POLLEN. [BOOK n. 



the doctrine of the sexuality of plants seemed finally estab- 

 lished, never again to be seriously controverted ; for it must 

 be admitted, that the denial of this fact, which has been 

 since occasionally made by such men as Alston, Smellie, and 

 Schelver, has carried no conviction with it. It is a general 

 law that the powder which is contained in the case of the 

 anthers, and which is called pollen, must come in contact 

 with the viscid surface of the stigma, or no fecundation can 

 take place. It is possible, indeed, without this happening, 

 that the fruit may increase in size, and that the seminal 

 integuments may even be greatly developed ; the elements of 

 all these parts existing before the action of the pollen can 

 take effect : but, under such circumstances, whatever may be 

 the development of either the pericarp or the seeds, no 

 embryo can be formed. This universality of sexes does not, 

 however, extend to cryptogamic plants, as has been already 

 shown. 



In order to insure the certain emission of the pollen at the 

 precise period when it is required, a beautiful contrivance is 

 provided. Purkinje has demonstrated the correctness of 

 MirbePs opinion in 1808, that the cause of the dehiscence of 

 the anther is its lining, consisting of cellular tissue, cut into 

 slits, and eminently hygrometrical. He shows that this 

 lining is composed of cellular tissue, chiefly of the fibrous 

 kind, which forms an infinite multitude of little springs, that 

 when dry, contract and pull back the valves of the anthers, 

 by a powerful accumulation of forces, individually scarcely 

 appreciable ; so that the opening of the anther is not a mere 

 act of chance, but the admirably contrived result of the 

 maturity of the pollen; an epoch at which the surround- 

 ing tissue is necessarily exhausted of its fluid, by the 

 force of endosmose exercised by each particular grain of 

 pollen. 



That this exhaustion of the circumambient tissue by the 

 endosmose of the pollen is not a mere hypothesis, has been 

 shown by Mirbel in a continuation of the memoir I have 

 already so often referred to. He finds that, on the one hand, 

 a great abundance of fluid is directed into the cells in which 



