FUNCTIONS OF HYDATHODES 499 



since been verified by Musset and Molisch ; tbe last-mentioned observer 

 has seen as many as 163 drops ejected in a single minute. The 

 significance of this process of " guttulation " is still obscure. According 

 to an observation made by the author, an adult leaf of Conocephalus 

 ovati/s, weighing 13*02 g., secreted 2'76 g. of liquid in a night, a 

 quantity equal to 26 per cent, of its fresh weight. 



The liquid secreted by hydathodes is naturally not pure water. 

 The solid matter present there is, however, as a rule quite small 

 in amount; it comprises both organic and inorganic compounds. 

 According to Unger, the liquid that escapes from the water-stomata of 

 Zea Mays contains "05 per cent, of solid matter, of which '027 per 

 cent, represents ash. The secretion of Colocasia antiquoriim contains 

 056 per cent, solids, but only "008 per cent, of ash, that of Brassica 

 cretica '1 per cent, of solids and "042 per cent, of ash. Van Eom- 

 burgh finally states that the secretion of Conocephalus ovatus, on being 

 dried at 100, leaves "045 per cent, of solid residue, which, when 

 calcined, is reduced to - 02 per cent, of ash. Evidently the liquid 

 secreted by leaves during the night contains no appreciable quantity of 

 nitrogenous or non-nitrogenous by-products of metabolism. 



In certain plants larger quantities of inorganic materials are 

 liberated along with the water secreted by the hydathodes. Thus 

 Wetter states that the secretion of the above-mentioned trichome- 

 hydathodes of Phaseolvs leaves nearly '5 per cent, of solid residue, 

 consisting principally of potassium carbonate. The same substance is 

 present in the liquid secreted by the very similar hydathodes that 

 occur on the leaves of various Malvaceae. In such cases, when the 

 water evaporates, the dissolved salts remain behind in the form of 

 minute crystals. The salts in question are very hygroscopic, and hence 

 deliquesce rapidly in moist air ; as a result the hydathodes may seem 

 to retain the power of secreting water even on severed leaves or leaf 

 fragments that is to say, independently of the general exudation- 

 pressure of the root or shoot. A similar phenomenon had previously 

 been noted by Volkens in the case of certain Tamarisuineae (Beaumurea 

 hirtella and Tamarix articulata), and of some other desert plants. Here 

 the saline efflorescence (consisting mainly of chlorides of sodium, 

 calcium and magnesium, but also including nitrates and plcospthates), 

 forms a veritable incrustation, which actively absorbs moisture from 

 the air at night. Volkens assumes, that the plant is able to withdraw 

 water from the resulting saline solution. The absorption of water by 

 living cells from a concentrated solution of salts, although not incon- 

 ceivable, certainly seems somewhat improbable. Marloth, on the other 

 baud, believes that the white saline crusts reduce the effects of insolation, 

 and also that the cooling effect due to the evaporation during the day of 



