HYDATHODES 487 



of secretory organs naturally varies according to the particular require- 

 ments in connection with which they are developed. 



Excretory reservoirs may occur as isolated idioblasts, or they may 

 be associated to form rows, or even more or less massive layers of cells. 

 In a number of cases fusion of originally independent cells takes place. 

 Often the completion of the excretory process is followed by the sub- 

 erisation of the walls of the reservoir, a modification which prevents 

 any subsequent effusion of noxious excretory substances ; in this way 

 the excretion is as effectually withdrawn from the metabolic cycle as 

 if it had been actually ejected from the plant-body. The arrangement 

 of excretory reservoirs is determined by a variety of considerations. 

 The most influential factor, of course, is the location of the particular 

 tissue or organ that is ultimately responsible for the production 

 of the excretory substance. It must, further, be of advantage to a 

 plant, if the excretory reservoirs are situated where they are least 

 likely to interfere with the physiological activities of the several tissues, 

 and with their mutual relations ; at the same time, it is important that 

 the excretory substances should be as far as possible eliminated from 

 the general metabolic cycle of the plant. Thus, excretory organs, especi- 

 ally when associated together in numbers, would be quite out of place, if 

 they were located at the surface of contact of two tissue-systems which 

 entertained an active intercourse with one another. Finally, the fact 

 that an excretory substance has an ecological value, may influence, or 

 even determine, the location of the organs in which it is produced. 



II SECRETORY ORGANS. 



A. HYDATHODES. 240 



Many of our native plants are provided with organs which secrete 

 water in the liquid form. Such hydathodes are even more widely 

 distributed among plants inhabiting the humid tropics. They are 

 most frequently located on leaves. As a rule, their activity only 

 begins when the hydrostatic pressure in the water-conducting system 

 (the so-called " root-pressure " or " exudation-pressure ") reaches a 

 certain intensity as a result of inhibited or reduced transpiration. 

 After a damp night, the leaves of hydathode-bearing plants are studded 

 with drops of water, each of which marks the position of a hydathode. 

 Formerly this secreted water was often confused with dew. 



1. Structure, and arrangement of hydathodes. 



The author has shown that the detailed structure of hydathodes 

 is subject to a considerable amount of variation. To begin with, we 



