278 



THE MOVEMENTS OF WATER 



Ballota nigra, Urtica dioiat, iVc., even when the water has only to be raised to 

 a height of a few centimetres ; in such cases it is easy by using a coloured 

 solution to make out the path which the water follows. The same physical and 

 structural peculiarities are concerned in this phenomenon, as render possible 

 the rapid drainage of rain-water from leaves. 



SECTION 48. The Excretion from Water-Pores. 



The leaves of many higher plants are able to excrete water in the 

 liquid form by means of special water-pores when in a saturated atmosphere 

 and when fully turgid. As a general rule the drops of water form directly 

 over the water-pores, which are usually situated upon the leaf-teeth (Fig. 38) 

 (luipaticns. Fuchsia, Tropaeohun, Vitis, Salix, &c.), but may occur in other 

 situations, as for example, on or near the tip of the leaf (in aroids and 

 grasses). When the pores are abundant and active, water may actually 



drip from the leaf. The filtration through 

 the external point of exit is usually only 

 passive, the cells which generate the 

 pressure of exudation being usually more 

 deeply seated. Hence the forcible injec- 

 tion of water induces a more rapid flow 

 through the water-stomata, while in the 

 day time the excretion of water usually 

 ceases, and even during the night it only 

 occurs when a sufficiently high exudation- 

 pressure is reached l . 



As might be expected, water-glands also exist in which there is an 

 excretion of water by the cells which line the cavity immediately beneath 

 the water-stoma, or in which we find only an acceleration of the outward 

 filtration of water due to the internal pressure of exudation. This is the 

 case, according to Haberlandt, in certain more or less superficially situated 

 water-glands (Conocephalns, Ficus), which yield a very watery fluid, and 

 which therefore very probably excrete water by means of the internal 

 activity of the cells of the water-gland. On the other hand, deep-seated 

 nectaries also exist in which the water is plasmolytically excreted. 



FlG. 38. Leaf with secreting water-pores. 



1 The experiments of Moll (Unters. tiber Tropfenausscheidung u. Injection von Blattern, 1880, 

 Scp.-abdr. a. Verslagen. en Meded. d. Kon. Akad. Amsterdam ; prel. comm. Bot. Zeitung, 1880, 

 p. 49) were followed by those of Volkens (Jahrb. d. Bot. Gartens in Berlin, 1883, Bd. II, p. i66)and 

 Gardiner <Troc. Camb. Phil. Soc., 1884, Bd. V, p. 35). A summary of the cases in which these and 

 other authors have detected a power of bleeding is given by \Vieler, Cohn's Beitrage, 1893, Bd. VI, 

 p. 16. See also Haberlandt (Sitzungsb. d. Wien. Akad., 1894, Bd. cm, Abth. i, p. 489, and 1895, 

 Bd. civ, Abth. i, p. 55 ; Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 511). That an excretion of water 

 might be caused by forcing water' into the plant was noticed by de Bary in 1869 (.Bot. Zeitung, p. 883). 

 and by Prantl (Flora, 1872, p. 381). 



