CIRCULATION OF THE SAP. 



147 



through the woody tissue, porous vessels, and cells, dissolving starch 

 and other matters, and appropriating various new substances. Pro- 

 ceeding upwards and outwards, this sap reaches the leaves, where it 

 is exposed to the air, and is elaborated by the function of respiration. 

 It then returns, or descends chiefly through the bark, either directly 

 or in a circuitous manner, communicating with the central parts by 

 the medullary rays, depositing various secretions, more especially in 

 the bark, and giving origin to substances which are destined to 

 nourish and form new tissues. Finally, it reaches the extremity of 

 the root, where absorption commenced ; a small portion is there 

 excreted, while the remainder mixes with the newly-absorbed fluids, 

 and again circulates in the sap. The rapidity with which the sap 

 ascends is dependent on the endosmotic property of the cells in the 

 roots, and on the density of the fluids. An absorption of water, con- 

 taining various matters in solution, is constantly going on through the 

 extremities of the rootlets. The sap thus formed is carried forward 

 through the cells, vessels, and intercellular passages, by a force which 

 acts by propulsion. The stimulus of light, acting on the cellular 

 tissue of the leaves, enables them to elaborate the organic compounds 

 which are necessary for vegetable nutrition. The leaf-action may be 

 reckoned one of attraction or suction, > transpiration giving rise to a 

 constant flow of fluids to supply the place of those exhaled. 



Dr. Pettigrew has given the following views as to the circulation 

 in plants, and has illustrated them in the accompanying diagram (fig. 

 242). In spring the sap being mainly concerned 

 in the growth of the branches, development of buds, 

 and evolution of leaves — a vigorous and rapid 

 movement takes place in an upward direction, 

 as at a. During summer, when the plant is 

 elaborating secretions, and storing up nourishment, 

 the course of the sap is partly upwards and partly 

 downwards, represented by the arrows at cd; the 

 ascending and descending currents are indicated as 

 continuous in the direction of the leaves and roots, 

 and thus as it were constituting a true circulation. 

 In autumn, owing to the fall of the leaf, excess 



and a general waning activity in the 



is a marked descent of the sap^ as 

 But besides, and consequent on, those main currents, 

 Thus the ascending spring and descending autumn 

 currents, being in great measure endosmotic, give rise to unequal 



Kg. 242. Diagram representing the ascending, descending, and transverse currents in the 

 plant, a. Ascending or spring current. 6, Descending or autumn current, c d, Ascending 

 and descending currents of summer ; these being continuous in the direction of the leaves 

 and roots, a c. Transverse currents. The aiTows in this diagram represent the endosmotic 

 currents, the darts the exosmotic ones. 



of moisture, 

 plant, there 

 shown at h. 

 others exist. 



Kg. 242. 



