THE CONDUCTION OF WATER. 



57 



collected in the course of a single night amounted, generally speaking, to about 

 10 g. and might reach as much as 22 g. The sap contains very few substances 

 in solution, for Berthelot, who at Duchartre's suggestion undertook an ana- 

 lysis of the fluid, was unable to do more than demonstrate the presence of the 

 merest traces of organic and inorganic materials in as much as 400 g. ; we may, 

 therefore, speak of the fluid, for our present purpose, as practically pure water. 

 The same phenomenon, differing from that seen in Colocasia only in degree, 

 occurs in many of our indigenous and cultivated plants. After a warm night, 

 small drops resembling dew are to be seen at the apices of leaves, on the leaf 

 teeth, and, more rarely, on other parts of the leaf. It is quite easy to show 

 that these are in no sense dewdrops, but are the result of the activity of the 

 plant itself, for they occur frequently only on the young leaves, while there 

 is no reason why dewdrops should not appear on the older leaves as well. The 

 drops increase gradually in size, fall off, and are once more replaced, but they 

 never aggregate into such quantities of fluid as are given off by Colocasia. 

 Well-known examples of the excretion of drops are furnished by the leaf apices 

 of grasses, the leaf teeth of Fuchsia, Alchemilla, Brassica, and the potato ; 

 while Tropaeolum and many Urticaceae and Moraceae give off water not only 

 from the edges of their leaves but from the surfaces as well. 



..^Sp 



Fig. 13. Water stoma of Vtcia faba 

 in section, sp, the stoma ; g, vessels 

 abutting on the intercellular spaces. 

 (After Haberlandt, 1895, pi. 3.) 



Fig. 14. Longitudinal sec- 

 tion through a leaf tooth of 

 Primula sinensis. .S/, stoma; 

 Ep, epithem; C, vessels. (After 

 Haberlandt, 1^95, pi. 4.) 



This excretion of water is effected by special organs, the so-called ' hyda- 

 thodes '. These differ, as a rule, from ordinary stomata in being larger and in 

 having immobile guard-cells ; for these reasons they have received the special 

 name of ' water-stomata '. They occur singly or in groups in the situations 

 where excretion of water takes place, and it is through them that the water 

 escapes which has accumulated in the underlying space corresponding to the 

 respiratory cavity of the ordinary stoma (compare p. 37). Usually the 

 ends of the vascular bundles only lie in close relation to these cavities. 

 In the simplest cases (grasses and Vicia sepium. Fig. 13) the ultimate tracheids 

 run immediately under the respiratory cavity, and frequently small superficial 

 tracheids border directly on this large intercellular space, or are separated 

 from it only by loosely arranged parenchymatous tissue ; the parenchyma 

 which immediately invests the vascular elements is practically destitute of 

 intercellular spaces. In the more highly developed organs which occur in 

 Fuchsia and Primula (Fig. 14), the tracheids open out at the ends of the bundle 

 in a brush-like manner, and the spaces between the individual tracheids and 

 also the not inconsiderable gap between the ends of the bundles and the water- 

 stomata are filled by parenchymatous cells. This parenchyma (epithem : Ep, 

 Fig. 14) is composed of cells which are much smaller than those of the meso- 

 phyll, and is not infrequently deliminated from it by a cuticularized sheath. 



