486 SECRETORY AND EXCRETORY SYSTEMS 



differentiates secretory organs from excretory reservoirs, consists in the 

 fact that the secretion is removed from the organ or cell in which it 

 is produced. 



The above comprehensive definition takes no account either of the 

 form of the secretory organ or of the mechanism of secretion. In the 

 sequel the term gland will be applied to those localised secretory organs 

 in which the production and liberation of the secretion is carried out 

 by the living protoplasts of the secretory cells. The substance secreted 

 by a gland may escape at once to the outside (as in the case of glandular 

 hydathodes and nectaries), or it may pass into a glandular cavity of one 

 kind or another. When the gland is superficial, the glandular cavity 

 usually owes its origin to a local separation of the cuticle from the rest 

 of the epidermal wall. In the case of internal glands, on the other 

 hand, the cavity corresponds to an intercellular space, which may arise 

 either by the separation or by the disintegration of the secretory cells. 

 It is accordingly customary to discriminate between schizogenous and 

 lysigenous glands, although the two types are connected by transitional 

 forms. 23 ''* 



The principal difference between excretory reservoirs and secretory 

 organs consists in the fact that the ceils of which the former consist, or 

 from which they develop, contain [and permanently retain] by-products 

 of metabolism, i.e. substances which are valueless from a nutritive point 

 of view. Excretory substances may secondarily acquire a considerable 

 amount of ecological importance (for example, as agents of protection 

 against animals); there are, however, good reasons for believing that many 

 processes of excretion merely serve to remove useless waste-products 

 from the photosynthetic and conducting system. It is, therefore, always 

 easy to distinguish between a secretory organ and an excretory reservoir. 

 There may, however, occasionally be some doubt as to whether a particular 

 cell should be assigned to the excretory or to the storage-system ; one 

 cannot, indeed, draw any sharp line of demarcation between useless 

 metabolic by-products and substances of metabolic origin which are 

 capable of further utilisation. Moreover, one and the same substance 

 (such as tannin) may in some cases represent a plastic material, while 

 in others it is purely excretory in nature. 



It is, naturally, impossible to make many general statements con- 

 cerning the structure of secretory and excretory organs. The simplest 

 types of secretory organ are unicellular ; more often, however, as has 

 already been stated, a larger or smaller number of secretory cells are 

 closely associated together. Since the actual secreting elements are thin- 

 walled and therefore exposed to injury, they are frequently provided with 

 arrangements for mechanical protection. Secretory cells generally 

 contain a well- developed protoplast and a large nucleus. The disposition 



