SECT. II 



PHYSIOLOGY 



215 



and phosphoric salts of the soil are not obtained by them in the same 



quantities as in the case of the more vigorously transpiring land-plants. 



This is very evidently the case 



in the Sundew (Drosera), which 



is loosely attached by a few roots 



upon a thick spongy carpet of 



Bog-moss, and must find in the 



animal food a valuable addition 



to its nitrogenous nourishment. 





Fro. 188. — A leaf of Drosera rotundifolia. The 

 stalked glands and their secretions serve for the 

 capture and digestion of insects. (After 

 Darwin, enlarged.) 



A great variety of contrivances for 

 the capture of insects are made use of 

 by carnivorous plants. The leaves of 

 Drosera are covered with stalk-like out- 

 growths ("tentacles"), the glandular 

 extremities of which discharge a viscid 

 acid secretion (Figs. 188 and 115). Any 

 small insect, or even larger fly or moth, 

 which comes in contact with any of the 

 tentacles is caught in the sticky secre- 

 tion, and in its ineffectual struggle to 

 free itself it only comes in contact with 

 other glands and is even more securely 

 held. Excited bythe contact stimulus, 

 all the other tentacles curve over and 

 close upon the captured insect, while 

 the leaf-lamina itself becomes concave 

 and surrounds the small prisoner more closely. The secretion is then discharged 

 more abundantly, and contains, in addition to an increased quantity of formic acid, 



a peptonising ferment. The im- 

 prisoned insect, becoming thus 

 completely covered with the secre- 

 tion, perishes. It is then slowly 

 digested, and, together with the 

 secretion itself, is absorbed by the 

 cells of the leaf. . 



In Pinguicula it is the leaf- 

 margins which fold over any small 

 insects that may be held by the 

 minute epidermal glands. In 

 species of Utricularia (Fig. 34), 

 growing frequently in stagnant 

 Pig. 189.-A leaf of Dwnaea musdpula, showing the water, small green bladders (meta- 

 sensitive bristles on its upper surface, which, in the morphosed leaf-tips) are found on 

 parts shaded, is also thickly beset with digestive ^ e tip S f the dissected leaves, 

 glands. (After Dabwin, enlarged.) In each bla( ider there is a small 



opening closed by an elastic valve which only opens inwards. Small snails and 

 crustaceans can readily pass through this opening, guided to it by special out- 

 growths ; but their egress is prevented by the trap-like action of the valve, so that 

 in one bladder as many as ten or twelve crustaceans will often be found imprisoned 



