G. M. Thomson. — On the Fertilization of Flowering Plants. 279 



lower side of the flap, which would not, however, receive any pollen during 

 the withdrawal. I think it quite impossible that this species can be self- 

 fertilized. From its habitat, standing, as the flowers do, right out of the 

 water, it is probably visited by small species of Diptera. 



As already mentioned, the name of this genus is derived from the bladders 

 (Lat. utriculus ) attached to the leaves or rhizomes. The primary function 

 of these bladders would appear to be that of floating the plant during the 

 flowering season to the top of the water in which it grows. The following 

 quotation from De CandoUe's " Vegetable Physiology " is extracted from 

 Le Maout and Decaisne's " System of Botany," p. 591 : — " These bladders 

 are rounded and furnished with a kind of moveable operculum. In the 

 young plant they are filled with a mucus heavier than water, and the plant, sub- 

 merged by this ballast, remains at the bottom. Towards the flowering season 

 the leaves secrete a gas which enters the utricles, raises the operculum, and 

 drives out the mucus, when the plant, now furnished with aerial bladders, 

 rises slowly and floats on the surface, and there flowers. This accomplished, 

 the leaves again secrete mucus, which replaces the air in the utricles, and 

 the plant re-descends to the bottom, and ripens its seeds in the place where 

 they are to be sown." 



This view of the function of the bladders may apply in the case of such 

 species as U. neglecta (of Europe), which bears them on the leaves, and is 

 quite destitute of roots ; but in the case of the species under consideration, 

 and all others which bear them on the subterranean rhizomes or creeping 

 stems, some other explanation must be sought. It is probable that the view 

 advanced by Darwin in his " Insectivorous Plants," p. 395, is the correct 

 one — that the bladders have now become adapted (whatever may have been 

 their original function) for the capture of small aquatic animals, and their 

 subsequent absorption. In the work named Darwin details very muiutely 

 the structure of the bladders of U. neglecta. These are remarkably well 

 adapted for the capture of prey, theu' aperture being furnished with bristles 

 directed into the interior of the cavity, hke a rat-trap. Entrance into the 

 bladder is easily accomplished, but, once in, escape is almost impossible. 

 The interior of the bladder is lined with what are termed " quadrifid pro- 

 cesses," which are four elongated cells borne almost crosswise from the 

 summit of a minute projection or foot-stalk. Besides these there are "bifid 

 processes," similarly placed on stalks, and various forms of glands. All 

 these bodies are found to have the power of absorbing decaying animal 

 matter, as well as weak solutions of salts of ammonia and urea. As the 

 older bladders are found to contain animal remains always more or less dis- 

 integrated, there seems little doubt that their presence must be beneficial to 

 the plant. As to what entices the animals entrapped to enter the bladders 



