PRINCIPAL, PARTIAL AND SUBSIDIARY FUNCTIONS 63 



by its mode of origin. The importance of each tissue in the economy 

 of the entire plant depends altogether upon its physiological value in 

 the adult condition, and is not in the least affected by its ontogenetic 

 development. 



B. THE PHYSIOLOGICAL FUNCTIONS OF TISSUES. 



The principal function of a tissue is that form of physiological 

 activity with which its most obvious and important anatomical features 

 are correlated. In this connection the term " anatomical features " 

 includes the topographical arrangement of each tissue in the various 

 organs of the plant-body, as well as the histological characters of its 

 component cells. The principal function of a tissue or tissue-system is 

 often compounded of several partial functions. In the case of the 

 epidermis, for example, the principal function consists in the protection 

 which this layer affords the plant against a variety of hostile environ- 

 mental factors. This general function of protection includes several 

 partial functions, such as prevention of excessive transpiration, restric- 

 tion of nocturnal radiation, protection against intense insolation, diminu- 

 tion of the risk of mechanical injury, etc., all of which are embodied in 

 the anatomical structure of the epidermis. Every epidermal cell betrays 

 in its histological peculiarities the influence of some of these partial 

 functions, if not of all. 2 ' In other cases the partial functions of a 

 complex tissue-system may be distributed among different component 

 elements, the combined activities of which constitute the principal 

 function of the system. In the case of the conducting system, for 

 instance, the principal function is conduction in general. The various 

 partial functions, such as transportation of water, conduction of carbo- 

 hydrates or translocation of protein-substances, are allocated to distinct 

 component tissues. The histological complexity of a tissue or tissue- 

 system is in fact largely determined by the number and variety of 

 the partial functions which it has to perform. 



There is rarely any fundamental difference of opinion with regard to 

 the principal function of a tissue, or any serious doubt as to the 

 anatomical characters which should serve to determine the nature of 

 this principal function. The conditions are quite different, however, 

 in the case of subsidiary functions and their anatomical criteria. It 

 is quite certain that such subsidiary functions must be of very wide- 

 spread occurrence. No one, at any rate, would venture to assert that a 

 given tissue could never be confronted with physiological demands other 

 than those normally connected with its principal function, even among 

 the most highly -organised plants, where internal differentiation has 

 been pushed to the furthest extent. For one thing an extreme degree 

 of differentiation in itself entails certain disadvantages in the case of 



