138 BIOLOGICAL CHEMISTRY 



Digestion in Plants and Animals other than Mammals. 

 The mechanisms described are very instructive as they 

 probably represent elaborations of mechanisms that occur 

 even in unicellular organisms. The regulation of enzymes 

 cannot be so satisfactorily demonstrated in simpler organisms 

 as in the cases where the gland cells are collected into definite 

 groups with special nerve supplies, and often a duct from which 

 the secretion can be obtained. Yet in seeds, plants, and the 

 simpler animals, there are evidences of the regulation of 

 enzyme action. Protozoa, for instance, secrete acid into their 

 digestive vacuoles. 



In addition to bacteria other plants have specialised 

 digestive secretions. The scutellum or other portions of the 

 seed furnish enzymes which dissolve the stored food. Enzymes 

 that act upon cellulose ; glucosides, such as salicin, etc., are 

 known in addition to those already described. 



Fungi and other saprophytic or parasitic plants digest 

 and absorb the organic matter upon which they feed. 

 Insectivorous plants show all stages from those which drown 

 insects and allow their bodies to undergo bacterial decom- 

 position to those which possess specialised digestive secretions 

 with some form of regulation so that the secretion is formed 

 only when required.* 



An interesting adaptation is that of some intestinal worms. 

 They do not need digestive secretions of their own as digested 

 products are formed in the intestine surrounding them, so 

 they merely absorb the food products furnished by the diges- 

 tive activity of their host. The worms are not themselves 

 digested, possibly because they contain an antitrypsin which 

 prevents the action of trypsin.f The alimentary canal may 

 be furnished with antienzymes ta prevent it digesting itself. 



There is no doubt that in the plant the materials formed in 

 the leaves are transferred by movements of liquid in the stem 

 in the form of simple soluble substances. Thus there must be 

 hydrolysis of the starch, etc., in the leaf. The soluble sub- 

 stances are transferred to food depots in the seeds, etc., and 

 turned into insoluble substances. When the seed commences 

 to grow the food materials are once more hydrolysed and 

 used as simple soluble substances. 



The same processes occur in each individual cell of plants 

 and animals. The simple soluble substances are dehydrated 

 to form insoluble colloidal substances which are the food 



* C. Darwin, Insectivorous Plants. Murray, 1875. 

 f E. Wcinland, Zeit. f. Biol., 1903, vol. 44, p. i. 



