680 PART IV. — THE PHYSIOLOGY OF PLANTS. 



which contain them {e.g. most Conifers, etc.), is opened by a 

 wound, the resin serves to protect the raw surface both mechani- 

 cally arid antiseptically ; and this doubtless also applies to the 

 lat^ex 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 pre- 

 sence 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, see p. 627) 

 a proteolytic ferment, and it has hence been inferred that this 

 tissue may serve to conduct plastic substances throughout the 

 plant ; but this inference has not been satisfactorily established. 



§ 5. The Functions of the Members. It has been pointed 

 out (p. 6) that, in its highest development, the plant-body consists 

 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. 63) : under these circumstances 

 other members become modified to perform the absorbent function of the root ; 

 in Salvinia, the aquatic leaves ; in Psilotuni, the subterranean shoots. In the 

 "carnivorous" plants {e.g. Drosera, Diontea, Nepenthes), though tbey possess 

 roots, the leaves are adapted for the absorption of organic food in solution. 



In a typical land-plant the development of the root-system is 

 such as to ensure an adequate supply of food from the soil, and a 

 supply of water sufficient to maintain the general turgidity of the 

 plant in spite of continued loss of water by transpiration. 



The root of such a plant is adapted for the performance of its 



