GENERAL VIEW 209 



The bleeding- or exudation-pressure l which not only roots, but stems also, 

 are able to generate, is usually less than that of one atmosphere, and hence 

 this pressure of exudation, even were it shown by all plants, which is not 

 the case, would be quite insufficient to explain the raising of water to the 

 summits of lofty trees. Moreover, it is just when the greatest quantities 

 of water are being carried upwards that no active pressure of exudation is 

 manifested. Similarly, herbaceous plants can fully supply themselves with 

 water without the aid of any 'bleeding-pressure,' although during damp 

 nights water frequently accumulates in them to a sufficient extent to render 

 an exudation of water possible. This is the case in Imp aliens ^ and in 

 certain Aroids and Grasses, as is shown by the escape of drops of water 

 from the leaf-teeth or other points, for this is dependent upon an over- 

 accumulation of the water driven forcibly upwards. All excretion of 

 water as such is not, however, necessarily connected with the existence 

 of a pressure of exudation, for fluid may still be excreted from nectaries 

 when the plant is suffering from drought. In this case the soluble 

 constituents of the nectar cause the excretion of the water, which, just 

 as in plasmolysis, must necessarily escape when a permeable tissue is 

 in contact on one surface with an osmotically active substance. The 

 exudation-pressure is, however, due to the water being driven in one 

 direction by the direct action of living cells. The details of this process, 

 as well as the relations between the exudation-pressure and the supply of 

 water, will be discussed in the different sections of this chapter. 



Vegetables, fruits, and succulent tissues in general contain 70 to 90 per cent, of 

 water, and in certain very fleshy and watery fruits as much as 95 per cent, may be 

 present. In turgid cells the cell-sap does not usually contain more than 3 to 6 

 per cent, of solids, while the protoplasm generally comprises 10 to 30 per cent. 

 (Sect. n). Soft cell- walls probably contain a similar amount of water, while 

 lignified walls when fully saturated hold about 50 per cent. (Sect. 12). As trees 

 grow older the percentage of dead cells increases, and these, according to circum- 

 stances, may be filled with air or water to a varying extent. Nevertheless, even in 

 periods of drought, the amount of water present in trees does not readily fall below 

 30 per cent, and may at times rise to as much as 70 per cent. 



Numerous data and comparisons of the amounts of water present in different 

 plants and different tissues are given by Ebermayer, Physiol. Chemie der Pflanzen, 

 1882, p. 2 ; J. Konig, Chemie der Nahrungs- und Genussmittel, 1889, Th. I, p. 641. 

 In these works, and in the treatises quoted therein, determinations are given of 

 the changes in the percentage of water as plants and organs grow older. Cf. also 

 Aubert, Ann. d. sci. nat., 1892, vii. se'r., T. xvi, p. 59. 



1 [For a definition of this term see p. 254.] 



