498 SECRETORY AND EXCRETORY SYSTEMS 



stomata. The path described evidently offers the smallest resistance 

 to filtration, and is doubtless that by which the secretion actually 

 escapes. The passive character of the epithem may be inferred from 

 the fact that, if the tissue in question is poisoned or put out of action 

 by anaesthesia or by exposure to extremes of temperature, artificial 

 pressure will induce a flow of water quite as readily as it does when 

 the hydathodes are in their normal condition. This conclusion was 

 arrived at by the author on the basis of some experiments performed 

 upon Fuchsia globosa ; in view of the results previously obtained by 

 Moll, who succeeded in forcing the red juice of Phytolacca and 

 a 1 per cent, solution of tannic acid through the leaf-teeth in various 

 plants, the same argument probably applies to the great majority of 

 plants that are provided with epithem-hydathodes. 



It now becomes necessary to explain the function of the epithem, 

 which is usually a very well-defined tissue. Wilson and Gardiner 

 noted that severed branches of Fuchsia, when kept in a moist atmosphere 

 with the cut ends immersed in water, secreted a small amount of liquid, 

 evidently as a result of active exudation on the part of the epithem 

 tissue. Although this process of active secretion is not great, it no doubt 

 suffices to keep the intercellular spaces of the hydathodes full of water, 

 and thus to preserve the water-conducting system from contact with the 

 outer air ; this, in fact, most probably constitutes the special function 

 of the epithem wherever the secretion of liquid water is merely a 

 process of filtration under pressure. In Conocephalus and probably in 

 other Moraceae also the power of active secretion possessed by the 

 epithem, which was originally feeble and of secondary importance, has 

 become accentuated to such an extent as to become responsible for the 

 entire water-secreting activity of the plant. It is not at all unlikely 

 that further investigation may reveal the existence of various transi- 

 tional stages between the types of hydathodes exemplified respec- 

 tively by Fuchsia and by Conocephalus. In Vicia sepium and in the 

 Gramineae, where epithem is absent, the process of secretion must 

 obviously consist entirely of filtration under pressure. 



The quantity of water secreted by hydathodes is often very con- 

 siderable. An adult leaf of Colocasia antiqiiorum, for example, was 

 found by Duchartre to secrete, on an average, between 9 and 12 g. of 

 water in the course of a night; on one occasion, however, it produced 

 as much as 22 6 g. Molisch estimated the amount of water secreted 

 in a single night by a young leaf of Colocasia nyniphaefolia as varying 

 from 48 to 97 c.c. In this plant the water exudes from a hydathode 

 situated immediately behind the tip of the leaf. Here the liquid is 

 actually ejected in drops, which follow one another in rapid succession. 

 This curious fact was first recorded by Muntingh in 1672, and lias 



