THE EXCRETION FROM WATER-PORES 279 



The excretion of water from water-pores is a typical process of 

 filtration under pressure, for as Moll has shown, no water escapes from 

 a cut branch however well supplied it may be, whereas the flow commences 

 as soon as water is forcibly injected. Similarly, Haberlandt found that 

 ordinary water-stomata continued to excrete after the epithelial tissue 

 through which filtration occurs had been killed by poison, whereas in 

 the glands of Conocephalus such treatment caused a cessation of the flow, 

 for in them the excretion of water is independent of the general internal 

 hydrostatic pressure J . 



The fact that the flow ceases as soon as a branch is cut off is a 

 sufficient proof that the excretion of water is due to the general exudation- 

 pressure, but if no water is allowed to escape from the cut surface of 

 the stem the water-stomata may remain functionally active so long as the 

 cut branch can maintain a sufficiently high internal hydrostatic pressure. 

 Cut stems may indeed in certain cases continue to excrete water, but it 

 does not follow that the process is independent of the general internal 

 pressure, so that the conclusions which Gardiner drew as to the active 

 excretion of water by the epithelial cells of certain water-pores are not 

 necessarily correct 2 . 



As regards the shape and anatomical structure 3 of the water-excreting organs, 

 it must suffice to say that for the most part they are in the form of the so-called 

 ' water-pores,' i. e. modified widely open stomata, beneath which a respiratory cavity 

 persists. This becomes filled with water driven into it by filtration through the inter- 

 mediary cells which are usually directly connected with a subjacent vascular bundle. 

 The intermediate tissue has somewhat the form of an epithelium, between the cells 

 of which small or large intercellular spaces occur, and these may aid in the escape 

 of water 4 . Moll observed an escape of water coloured with dye from the water- 

 pores of Phytolacca decandra when it was forced into the plant by pressure, but the 



1 Haberlandt, I.e., 1894, p. 513; JS95, pp. 63,81, 107. [Spanjer denies that water is ever 

 actively secreted through glandular hydathodes, and states that the escape of water is always due to 

 filtration under the generally distributed internal hydrostatic pressure (Bot. Zeitung, 1898, Nos. 3 

 and 4). It is worthy of notice that the latter is very marked in Conocephalus. No final decision 

 is, however, as yet possible. Cf. also I.e., pp. 177, 241, 315.] 



- Gardiner, I.e. p. 14; cf. also C. Kraus, Studien a.d. Geb.d. Agriculturphysik, 1882, Bd.v,p.435. 

 3 For details see the above works. Cf. also de Bary, Anat., 1877, pp. 54, 391 ; Haberlandt. 



Physiol. Anat., 1896, 2. Aufl., p. 415; Nestler, Unters. iiber die sog. Wasserspalten, 1894 (Sep.-abdr. 

 a. Nova Acta d. Leopoldina) ; Nestler, Sitzungsb. d. "Wien. Akad., 1896, Bd. cv, Abth. i, p. 521. 

 [Max v. Minden, Beitr. z. anat. u. physiol. Kenntniss \Yassersecernierender Organe, Bibl. Bot. 

 1899. Heft 46, pp. 1-71.] 



* Haberlandt (I.e., 1895, p. 107) makes a physical error in supposing that the high osmotic 

 pressure existing in the epithelial cells will hinder the filtration of water through them. Both the 

 osmotic absorption and the excretion of water are simply processes of filtration under pressure, and 

 the one may replace the other. Moreover, the facts observed do not justify Haberlandt's conclusion 

 (I.e., 1895, p. no) that the stimulating action of a high internal hydrostatic pressure is necessary in 

 order to cause an excretion of water from glands which have an intracellular excretory activity of 

 their own, for the stoppage of all excretion of water is a mechanical necessity when the amount 

 present falls below a certain limit. 



