LEAVES 



619 



secretion of enzyms and of the digestion of animal food. Bladders may assist in 

 the flotation of the plant, though in such a role the eel-trap structures can have no 

 significance ; it may be noted that bladders occur, though less abundantly, in land 

 species. Possibly the bladders have no role of importance. Insects are held by 

 viscid secretions on the stems of Silene antirrhina (hence called catchfly), and 

 are drowned in the water-containing leaf cups of Silphium perfoliatum and Dip- 



sacus sylvestris, but in none of these 

 cases is there evidence of enzym se- 

 cretion or of food digestion. 



FIG. 909. A leaf of a bladderwort 

 (Utricularia vulgaris), showing numerous 

 capillary divisions, many of which bear 

 bladders (>), especially near the place of 

 attachment to the main leaf axis (a); note 

 the apertures (p] of the bladders into which 

 small aquatic animals may crawl or swim. 



FIG. 910. A longitudinal section 

 through the bladder of a bladderwort 

 (Utricularia neglecta), showing long ex- 

 ternal hairs (h) about the entrance, an 

 elastic valve (-y) which entering animals 

 readily push back, a cushion (c~) on which 

 the valve rests, and the interior cavity of 

 the bladder (i) in which the animals re- 

 main imprisoned ; the cavity is lined with 

 small branched hairs (s), the so-called 

 absorptive organs of the bladder; con- 

 siderably magnified. From KERNER. 



The significance of the carnivorous habit. The general restric- 

 tion of carnivorous plants to bogs has led to the view that the car- 

 nivorous habit is advantageous in supplementing the nitrogen supply, 

 which has been thought inadequate in such habitats. However, 

 the vast majority of bog plants have no such unusual method 

 of getting nitrogenous food, and yet they thrive as well as or 

 better than do the carnivores. Even in the sundew the advan- 

 tage of animal food, so far as known, is slight, and in other plants 

 the proof of such advantage is wanting. In all carnivorous plants 

 animal food probably is a comparatively incidental feature of nutri- 

 tion. In view of the foregoing, the question of the evolution of 

 the carnivorous habit arouses much interest. It has been thought 



