viii] THE BLADDERS OF THE BLADDERWORTS 93 



U. vulgaris in Europe. Darwin ^ describes the valve of Utricu- 

 laria neglecta as attached on all sides to the bladder, excepting 

 by its posterior margin, which is free and forms one edge of 

 the slit-like orifice. This margin is sharp, thin and smooth, 

 and rests on the edge of a rim or collar which projects into the 

 interior of the bladder. The collar obstructs any outward move- 

 ment, with the result that the valve can 

 only open inwards. The function of the 

 bladders was for a long time in dis- 

 pute. Certain ingenious but mistaken 

 theorisers regarded the little four-armed 

 hairs (Fig. 61), which occur within the 

 bladders, as root-hairs, and supposed 

 that the bladders existed in order to pro- 

 tect these delicate organs from the direct ^^^- ^^- utricuiariaBremii, 



'^ . Heer. Glands from the in- 



action of light and the depredations of tenor of a bladder. [Meier- 

 Crustacea 2! On a more plausible view, ''^^"' ^- ('902)0 

 it was maintained that the bladders were to be interpreted as 

 floats, which buoyed up the plant in the water. This idea has been 

 discounted, however, since many terrestrial Utricularias produce 

 large numbers of bladders; moreover it has been shown that 

 the Utricularias do not sink when all the bladders are removed^. 

 A third hypothesis now holds the field namely, that the 

 bladders act as traps for small animals which serve as food for 

 the plant; this theory may now be considered to be fully proved. 

 Before the middle of the last century, Treviranus* had recorded 

 the finding of a beetle and some small snails in the bladder 

 of a terrestrial Vtricularia (U. Hookeri) and had suggested the 

 comparison between these organs and the pitchers of Sarra- 

 cenia^ Nepenthes, and other carnivorous plants. But it was not 

 until 1875 ^h^t the fact that our native Utricularias preyed 

 on small animals was definitely proved. In this year Cohn^ 



1 Darwin, C. (1875). 2 Crouan (Freres) (1858). 



3 Darwin, C. (1875), Busgen, M. (1888), Goebel, K. (18892) and 

 (i 891-1893). 4 Treviranus, L. C. (1848I). 



5 Cohn, F. (1875). See also Darwin, C. (1875). 



