40] CHAPTER I. GENERAL PHYSIOLOGY. 163 



The absorptive function is confined to the primary tegumentary 

 tissue : it is by means of this tissue that absorption is carried 

 on by subterranean roots, either with or without root-hairs (see 

 pp. 109, 110), as also by the general surf ace- of submerged parts 

 of aquatic plants (p. 109). 



The prevention of excessive transpiration is effected by the 

 more or less well-marked cuticularisation of the walls of the 

 epidermal and peridermal cells of sub-aerial parts. Since these 

 walls, though more or less pervious to gases, are almost or 

 altogether impervious to watery vapour, the watery vapour 

 evolved in the interior of the plant has to escape through special 

 apertures, namely the stomata and the lenticels : and the tran- 

 spiration is further regulated (see p. 108) by the opening and 

 closing of the stomata. The importance of the tegumentary tissue 

 in preventing desiccation is directly established by the fact that 

 parts of plants deprived of their tegumentary tissue quickly dry 

 up : and indirectly, by the relation between the degree of develop- 

 ment of this tissue and the conditions of life of the plant. Thus, 

 this tissue is highly developed in plants which grow in dry 

 situations, whereas in the submerged parts of aquatic plants it 

 is imperfectly differentiated, and there are usually no stomata 

 or lenticels : hence, the more the conditions of life tend to pro- 

 mote transpiration, the more highly-developed is the tegumentary 

 tissue. 



b. The Parenchymatous Tissue (see p. 90), consisting as it i 

 typically does of cells which contain living protoplasm, is the 

 seat, not only of the metabolic processes, but also of the movements 

 and irritability of plants. 



Different nutritive functions are discharged by various regions 

 of this tissue. For instance, the parenchymatous tissue of sub- 

 aerial parts, lying near the surface and exposed to light, contains 

 chlorophyll, and carries on the assimilation of carbon : this applies 

 especially to the leaves. Again, the cells of this tissue are 

 frequently glandular (see p. 96), containing or excreting various 

 waste-products : or they serve as depositories of reserve plastic 

 substances (e.g. starch, etc.), or as conducting-tissue for organic 

 substances. 



Further, the cells of this tissue, having usually extensible 

 walls, are capable of becoming turgid and of varying in bulk : hence 

 they are the seat of the movements of those members, or parts of 

 them, in which movement is a mechanical possibility ; and when 



