RELATION TO ENVIRONMENT 509 



of tendrils (Fig. 476), but their function is the same as those of 

 Sarracenia. 



The Droseraceae comprise the well-known Sundews (Drosera), and 

 the monotypic Venus's Fly-trap (Dioncea muscipuld), of the bogs of 

 North Carolina. In Drosera (Fig. 477) the leaves are either linear or 

 spoon-shaped, in either case being more or less completely covered 

 with reddish tentacles terminating in a knob which secretes a glisten- 

 ing viscid fluid. When a small insect comes in contact with these, it 

 is held by the sticky secretion, and at the same time the leaf begins 

 to wrap itself round its victim, which is soon rendered quite helpless, 

 and finally killed. From small glands between the tentacles a 

 digestive fluid is secreted, which is quite similar in its action to the 

 gastric juice of animals, so that here there is a true digestion. 



In Dionaea, the blade of the leaf is divided into two wings with 

 spiny margins, and suggests a steel trap. Upon the upper surface 

 of each half are three stiff hairs, which are sensitive, and when 

 touched, as happens when an insect alights upon them, the two wings 

 of the leaf close, so as to catch the insect in the trap, when it is 

 digested by the action of the secretion from the surface glands. 



Of the Utriculariaceae, Pinguicula has glandular leaves, which be- 

 have much like those of Drosera, but the various species of Utricu- 

 laria and Aldrovanda, which are aquatics, have little bladders upon 

 the segments of the finely cut leaves, and these act as traps for 

 small Crustacea, and, it is said, for very young fish. The trap has the 

 opening small, with the margins bent inward, so as to make ingress 

 easy, but egress almost impossible. So far as known, there is no 

 trace of a digestive process here, but the products of decomposition 

 are absorbed, and help to supply nitrogenous food. These carnivo- 

 rous plants are mostly either bog-plants or aquatics, and the root 

 development is usually deficient. Their peculiar habits are probably 

 to be attributed to an effort to obtain nitrogenous food. 



SYMBIOSIS' 



By symbiosis is meant the association of two organisms, in a 

 manner beneficial to both. The symbionts may be an animal and 

 plant, but .usually they are both plants. Of the first the best-known 

 are the cases among various low aquatic animals, like Hydra, some 

 species of Vorticella, Paramoecium, and other Infusoria, Spongilla, 

 etc., in which very minute Algae live within the bodies of these 

 animals, which presumably derive from their assimilative activity 

 certain food elements, giving in exchange shelter and probably 

 nitrogenous food. The case of the Lichens, where an Alga and 

 a Fungus are associated together, has already been discussed in a 

 previous chapter. 



