CHAP. VI. FERTILISATION. 281 



trace the progress of discovery of the precise nature of the 

 several parts of the stamens and pistil. Suffice it to say that, 

 in the hands of Linnaeus, the doctrine of the sexuality of plants 

 was finally established, never again to be seriously contro- 

 verted; for the denial of this fact, which has been since 

 occasionally made by a few men, such as Alston, Smellie, and 

 Schelver, has merely exposed the weakness of such hyper- 

 critics. We know that the powder which is contained in the 

 case of the anthers, and which is called pollen, must generally 

 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, what- 

 ever may be the developement of either the pericarp or the 

 seeds, no embryo can be formed. This universality of sexes 

 in vegetables, must not, however, be supposed to extend further 

 than what are usually called, chiefly from that circumstance, 

 perfect plants. In cryptogamic plants, beginning with ferns, 

 and proceeding downwards to fvxngi, there are either no 

 sexual organs whatever, or the males are so imperfectly deve- 

 loped as to be invisible, or of no effect. 



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

 precise period when it is required, a beautiful contrivance has 

 been prepared. Purkinge has demonstrated the correctness of 

 Mirbel's 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, which are 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 sur- 

 rounding 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 



