THE EXCRETION OF WATER FROM UNINJURED PLANTS 277 



and rhizomes to penetrate it more easily. In some cases, however, the 

 excretion of water is an unavoidable accompaniment to more important 

 phenomena, as for example the contraction of a protoplast which is going 

 to form spores, or the escape of water from the stimulated cells of a con- 

 tractile tissue. 



The function of the water-stomata is to get rid of superfluous 

 water, and so to avoid any pronounced injection of the intercellular spaces 

 with fluid, such as a high exudation-pressure might cause 1 . When the latter 

 is maintained for some time, however, the intercellular spaces gradually 

 become filled with water in spite of the presence of water-stomata, as 

 Moll found (I.e., p. 73); under normal conditions they generally remain 

 filled with air or are only temporarily and partially injected with water. 

 The means by which the intercellular system is kept free from water have 

 already been mentioned (Sects. 43 and 46), and, moreover, the backward 

 filtration through the root, which may occur when the exudation-pressure is 

 very high, acts as a regulatory check upon the over accumulation of water. 



The amount of water which the water-stomata excrete is in general 

 not sufficiently great to accelerate the transpiratory current 2 to any par- 

 ticular extent ; even in moist climates this stream is sufficiently strong 

 to provide an adequate supply of the nutrient salts required (cf. Sect. 38). 

 Nor can the excretion of water serve as a general means by which excreted 

 products may be thrown off, for it may happen that the functional activity 

 of the water-stomata, &c., is never called into play. The fact that cal- 

 careous deposits are formed only on certain plants shows that their appear- 

 ance is due to some specific peculiarity of the plants in question (Sect. 23). 



Since the regions where water exudes must be readily permeable, 

 they are naturally also better able to absorb water than the relatively 

 impermeable epidermis, when an external supply is presented to the 

 water-excreting organs of a flaccid plant 3 . 



A formation of dewdrops by the condensation of water is not the same 

 thing as an excretion of water by the plant, and a fertile source of error lies in 

 the fact that drops of water may be caused to collect upon and fall from the leaf 

 by purely physical agencies. Arendt* has shown that with a particular kind of 

 surface a capillary ascent of water upon the stems and leaf-stalks is possible, and 

 so by spreading along the veins a regular series of drops of water may collect 

 and fall from the tips of hanging leaves. This takes place in Leonurus cardiaca, 



1 Moll, Unters. iiber Tropfenausscheidung u. Injection, 1880, pp. 81 , 101 ; Gardiner, Proceedings 

 of the Camb. Phil. Soc., 1884, Vol. V, p. 42 ; Haberlandt, Sitzungsb. d. Wien. Akad., 1895, Vol. CIV, 

 Abth. i, p. in. 



a Cf. Haberlandt, 1. c., p. ill ; Burgerstein, Ber. d. Bot. Ges., 1897, p. 155. See also Sect. 38. 



3 Cf. Sect. 27. This is therefore by no means a specific peculiarity of certain plants, as Haber- 

 landt supposes (Sitzungsb. d. Wien. Akad., 1894, Bd. cm, Abth. i, p. 494). In the same way such 

 absorbent organs as the roots are able to excrete water under certain conditions. 



4 Arendt, Flora, 1843, p. 152; Stahl, Regenfall u. Blattgestalt, 1893, p. 112. 



