568 PHYSIOLOGY. 



a still greater force, as, in one case, in a Vine he found the force of ascent 

 sufficient to balance a column of water 48-50 feet in height. The quantity 

 obtained from a Birch tree 75 feet high, from November to May, was 

 1486 lb., in one day 63 Ib. being collected. The force and amount are 

 subject to diurnal variations. It is evident that the spring current, at 

 least, is partly owing to absorption by the roots, in the cells of which 

 decomposition and solution of starch are effected, and which must in con- 

 sequence absorb water greedily ; the engorgement of the tissues may cause 

 the liquids to be forced into and upwards along the course of the vessels 

 and ducts. 



In woody stems osmose also comes into play in conjunction with capil- 

 lary action and pressure dependent on the various causes before named. 

 Pressure resulting from increased temperature is illustrated by the cir- 

 cumstance that the flow of sap in the trunks of trees is greatest during the 

 daytime, when the trunk absorbs the sun's heat by its rough surface, and 

 least at night, when the tree is cooled by radiation. In the leaves the 

 transpiration and the movements effected by the wind afford the main 

 causes for the rush of sap. In the expanding leaf-buds, and in all por- 

 tions of the plant where vegetation is going on actively and where in con- 

 sequence large quantities of nutriment are required, the chemical trans- 

 formation of the cell-contents, which renders them available for nutri- 

 tive purposes, necessitates a large quantity of water ; and in consequence 

 an endosmotic current is produced. This chemical action does not neces- 

 sarily occur at the very point where growth is most active, generally, in- 

 deed, elsewhere, in what may be termed the store-cells, so that a current 

 is determined from the store-cells to the growing points. 



The transfers just alluded to may be compared to a row of firemen 

 handing on pails of water, in the absence of a hose or continuous pipe, 

 such as is represented by the blood-vessels of an animal. 



The spring ascent of sap in Dicotyledons is partly to be accounted for 

 by the solution of starch, or the decomposition of fixed oil &c., in the buds 

 and cambium-region, as above mentioned (just as occurs in the root or in 

 a seed beginning to germinate). But, as has been observed by VonMohl, 

 the inspissated juices thus formed do not lie in the sap-wood wherein the 

 ascending current flows, but in the cambium-layers, where the elaborated 

 sap descends ; and it is not clear why the ascending fluid, if moved by 

 endosrnose alone, does not pass out laterally into the cambium as soon a*s 

 it reaches the stem. That the buds, however, do exert this attractive 

 force is seen by the influence of the heat of a greenhouse in causing the 

 flow of sap in a Vine which is planted with its roots outside the house, 

 and its stem brought inside and trained there. 



Transpiration. In the leaves (and green portions of plants 

 generally) the very important phenomena of evaporation or trans- 

 piration o watery vapours occurs, and constitutes probably the 

 most important agent of all in causing the supply and diffusion of 

 food in plants. It has been stated above that plants absorb their 

 liquid food by their roots ; therefore, under equal external condi- 

 tions, a plant should receive the nutrient matters derived from its 

 liquid food in the ratio of the quantity of water passing through 

 its tissues and evaporated from its leaves &c., since the water 



