THE EXCRETION OF WATER FROM UNINJURED PLANTS 273 



pores, &c. is due to the production of a high exudation-pressure. Under 

 these circumstances the channels or intercellular spaces through which the 

 water flows, as well as the internal or external point of exit, have a purely 

 passive function to perform in conducting away the excreted water. The 

 function of the channels is the same whether the water is passively excreted, 

 or whether it is driven onwards by the pressure due to the pumping activity 

 of special water-excreting cells. The cells which excrete the water may be 

 near to or far removed from the spot where it ultimately appears, as may 

 also be the case in normal bleeding. The character of the conducting 

 channels, as well as of the pores by which it escapes, if any exist, may 

 markedly influence the rate at which an escape of water is possible, but 

 nevertheless the cause of its escape in all cases is directly or indirectly the 

 excretory activity of living cells. The sinking of such cells in a pit-like 

 depression is the first step towards the formation of a water-gland with 

 a well-defined duct or exit passage. At first the secreting cells may line 

 the entire space thus enclosed, and only later become restricted to its base. 



These considerations suffice to make it clear that no sharp distinction 

 can be drawn between the excretion of water from injured and from un- 

 injured organs, for in both cases the water escapes from special points only 

 because of the pressure of exudation which forces it onwards. Indeed, 

 in certain cases the normal progress of development may cause external 

 parts to be ruptured or thrown off, and so produce points at which an active 

 outward filtration of water is possible. The drops of water which appear 

 on the buds of the hornbeam and other trees are produced by the internal 

 exudation-pressure driving water through the scar formed when last year's 

 leaf was disarticulated l . In many leaves again the openings through which 

 water can escape are produced by rupture, and in the same way water- 

 stomata may be mechanically enlarged or widened (Sect. 48). 



The mode in which water may escape through open channels to the 

 exterior has already been discussed. The excretion of water through the 

 water-stomata often found on leaves is due in some cases to a generally 

 distributed hydrostatic pressure, but in others to an active excretion of 

 water from special cells bordering upon the cavity beneath the water- 

 stoma. The excretion of water from typical nectaries, on the other hand, 

 is plasmolytic in nature, but as a deep-seated position, such as occurs in 

 many nectaries, is also possible, the fluid may escape to the exterior by 

 a pore or duct. In water-pores and nectaries _two different types of 

 excretory mechanisms are exhibited, as will be made still more clear by 

 the special account of each given later (Sects. 48 and 49). It is not 

 necessary to discuss the distinctive means by which every excretion of 



1 Strasburger, Ban u. Verricht. d. Leitungsbahnen, 1891, p. 841. Mere also the related literature 

 is quoted (Th. Hartig. Anat. u. Physiol. d. Hokpflanzen. 1878. p. 347, c.V 

 PFEFFER T 



