CHAPTER I.. GENERAL PHYSIOLOGY. 675 



cell-sap, or of the cell-wall, of the constituent cells. The follow- 

 ing remarks apply especially to the terrestrial higher plants. 



a. The Tegumentary Tissue (pp. 132, 153, 208), whether pri- 

 mary (epidermis) or secondary (periderm), has as its primary 

 function the mechanical protection of the underlying tissues : but 

 it has the further functions of absorption and of preventing 

 excessive transpiration. 



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. 154, 159), as also by the general surface of submerged parts 

 of aquatic plants (p. 157). 



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. 156) 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 promote 

 transpiration, the more highly-developed is the tegumentary tissue. 



The epidermis of sub-aerial parts also produces hairs of various 

 kinds. The function of these is often secretory, indicating the 

 importance of the epidermis as a glandular tissue (see p. 142). 

 But, more commonly, they are purely protective, serving to 

 diminish transpiration and radiation, and to screen the chlorophyll 

 from too intense light. A clothing of hairs is characteristic of 

 xerophilous plants ; plants, that is, which habitually grow in dry 

 soil and sunny situations. 



The primary tegumentary tissue is also of importance in con- 

 nection with reproduction, as it sometimes gives rise to repro- 

 ductive organs (e.g. sporangia of Leptosporangiate Filicinae). 



