DIFPUSION OF FLUIDS. 567 



spring, consists principally of water pumped up from the roots to supply 

 the requirements of the growing cells, and the excess of it is evaporated 

 when the leaves expand ; but though mainly watery, it contains some 

 mineral matters, and also some ingredients which must be derived from 

 the action of the leaves of the preceding season. 



The upward direction of the watery sap, therefore, and still more the 

 downward current of the elaborated sap, must be understood in a general 

 sense as indicating the prevailing direction of the currents. A more 

 strictly correct expression would be to say that the sap, including all the 

 liquid nutritive juices of the plant, moves in the direction in which cir- 

 cumstances are most favourable to its flow, and to those spots where the 

 sap is most needed for the nutritive processes of the plant, or for purposes 

 of storage, as will be more fully explained in succeeding paragraphs. 



Causes of Ascent. The causes producing the ascent of the sap 

 are manifold. They vary not only in their nature, but, at different 

 times, in different .parts of the same plant and under varying cir- 

 cumstances. They act also separately or in conjunction. We will 

 first of all allude to the inducing causes separately, and then indi- 

 cate how, when, and where they act. Emlosmotic action consequent 

 on the absorption of fluids by the root is on all hands admitted to 

 play the principal share in the diffusion of fluids throughout the 

 plant. Capillary action and Imbibition facilitate the upward pas- 

 sage in or between the iibro-vascular tissues. Pressure, whether 

 exerted by the tension of the cell-walls upon their contents, and 

 itself consequent on endosmosis, or as the result of increased tem- 

 perature, which expands the air in the stem, forces the fluids to 

 move in the direction of least resistance. The oscillations produced 

 by the swaying of the branches, petioles, &c. by the wind also 

 occasion intermittent pressure, to which Mr. Herbert Spencer 

 attributes an upward thrust of the sap towards the point of least 

 obstruction. 



The profuse evaporation or transpiration of watery vapour from 

 the leaves is a powerful agent in producing an upward flow of fluid 

 to replace that which is lost in the manner indicated. The extra- 

 vasation or exudation of sap consequent on the mechanical strains 

 effected by the wind also give rise to a current of sap from below. 

 Chemical actions, such as the transformation uf starch into sugar 

 &c., necessitate a supply of water and create osmotic currents of 

 that fluid. 



Force of upward current. The roots take an important share in pro- 

 moting the upward flow of the spring sap. If, in spring, we notice the 

 surface of stumps of timber-trees which have been sawn off" in the pre- 

 ceding autumn, we find the cut surfaces wet with abundant exudation 

 from the outer layers of the wood ; and experiments made upon the cut 

 ends of branches, by Hales and others, show that the sap rises in them 

 with very considerable force in the case of the Vine, supporting a column 

 of mercury 26 inches in height. Clarke's more recent researches indicate 



