41] CHAPTER I. GENERAL PHYSIOLOGY. 167 



their deposit at or near the surface serves the purpose of protection 

 in various ways. For instance, the secretion of wax on the sur- 

 face is an obvious protection against wet. Similarly there can be 

 little doubt that when the system of resin-ducts, in plants which 

 contain them (e.g. most Conifers, etc.), is opened by a wound, the 

 resin serves to protect the raw surface both mechanically and 

 antiseptically ; and this doubtless also applies to the latex present 

 in many plants. Further, these waste-products, by their bitter, 

 acrid, or astringent taste, by their frequently poisonous properties 

 (e.g. alkaloids), or by their hardness, serve to protect the plants 

 from being eaten by animals ; for instance, the presence of raphides, 

 or of strongly acid sap, in the cells of leaves, etc., has been proved 

 to protect them against the attacks of snails. The secretion of 

 mucilage by the glandular hairs (colleters) often developed near 

 the growing-points of stems and leaves, serves to keep the young 

 tissues moist. 



The special functional importance of the laticiferous tissue is not 

 fully understood. There is no doubt that it is, in the first place, 

 a reservoir of waste-products, since the latex generally consists 

 largely of such substances (e.g. caoutchouc, as in Siphonia elastica ; 

 alkaloids, as in the opium of the Poppy, etc.). But the latex has 

 also been found to contain plastic substances, such as proteids and 

 carbohydrates, and in one case (the Papaw) a proteolytic enzyme, 

 and it has hence been inferred that this tissue may serve to con- 

 duct plastic substances throughout the plant ; but this inference 

 has not been satisfactorily established. 



41. The Functions of the Members. It has been pointed 

 out (p. 3) that, in its highest development, the plant-body con- 

 sists of the following members : root, stem, leaf. These members 

 will now be considered from the physiological point of view. 



a. THE ROOT. The most general of the functions of the root 

 is that it absorbs the solid food of the plant in solution from the 

 substratum, whatever it may be, on which the plant is growing ; 

 and that, at the same time, it acts as an organ of attachment : in 

 submerged plants the latter is its main use. 



In some few cases the plant is rootless (p. 44): under these circum- 

 stances other members become modified to perform the absorbent function 

 of the root ; in Salvinia, the aquatic leaves ; in Psilotum, the subterranean 

 shoots. In the " carnivorous " plants (e.g. Drosera, Dionsea, Nepenthes), 

 though they possess roots, the leaves are adapted for the absorption of 

 organic food in solution. 



