FUNCTIONS OF HYDATHODES 497 



2. Function and importance of hydathodes. 



From a physiological point of view, it is convenient to distinguish 

 between two different types of hydathode ; in one case the secretion of 

 water is a mere filtration under pressure, in the other the hydathode 

 itself takes an active part in the process. 



The category of active hydathodes comprises all epidermal water- 

 secreting organs (including both the unicellular and the multicellular 

 or trichome types among Angiosperms, as well as the water-glands of 

 Ferns), and also the epithem-hydathodes of Conocephalus and Ficus 

 (and perhaps also those of other Moraceae and Urticaceae). In all 

 these cases the energy employed in the process of secretion is supplied 

 by the glandular hydathode cells themselves. It is they that develop 

 the force which causes water to exude. The exudation-pressure in the 

 water-conducting system merely supplies the stimulus which sets the 

 pumping action of the hydathodes going. 



The author succeeded in proving experimentally that the hydathodes 

 in question are really active water-glands comparable to the sweat- 

 glands of animals by painting the surfaces of leaves provided with such 

 organs with a weak ("1 per cent.) alcoholic solution of mercuric chloride, a 

 procedure which kills the living cells of the hydathodes. If subsequently 

 to this treatment the natural exudation-pressure in the water-conducting 

 system rises, or if it is replaced by artificial pressure according to the 

 methods of De Bary, Moll, etc. (the severed branch being attached to 

 the shorter arm of a J -tube, and water forced into it under the pressure 

 of 10 to 40 cm. of mercury), no liquid escapes from those leaves 

 or leaf-regions on which the hydathodes have been killed. If only a 

 portion of the leaf-surface is painted with alcoholic corrosive sublimate, 

 an increased flow of water will be found to take place from the un- 

 injured hydathodes. When, on the other hand, all the hydathodes on 

 a leaf are poisoned, water-secretion stops altogether, and the inter- 

 cellular spaces of the mesophyll become more or less completely blocked 

 with water. 



fn the case of epithem-hydathodes which are provided with water- 

 stomata, the process of secretion as a rule consists in a simple filtration, the 

 requisite energy being provided by root-pressure, or, in more general terms, 

 by the exudation-pressure which may be set up in stems and branches 

 as well as in the root system. Here the hydathodes represent little 

 more than points of least resistance to filtration. The anatomical 

 structure of the epithem indicates the course followed by the water 

 after it leaves the distal end of the terminal tracheides ; some of the 

 latter abut directly upon water-containing intercellular spaces, which 

 in their turn communicate with the chambers underlying the water- 



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