CIRCULATION. 149 



475. Returning, the true sap distributes its treasures in due 

 and exact proportion as needed to every organ. Its course lies 

 in the tissues of the bark, cellular and woody, first distributed 

 over the under surface of the leaves, thence by the leaf-stalks 

 into the liber, and so pervading all, down to the extremities of 

 the roots. 



476. On its passage it makes deposits of food, first in the cells. 

 of the pith at the base of every incipient bud ; then in the cam- 

 bium regiou a copious store; next in the medullary rays a due 

 portion, some carried outward for the supply of the cortical 

 layer, and some inward for solidifying the wood ; and lastly, the 

 residue, often the richest legacy of all, falls to the root, and fills 

 every branch and fibre, however vast its extent. This last de- 

 posit is that which is first met and dissolved by the rising tide 

 of fluid in the following Spring. 



477. Growth progresses downward. Since the flowing 

 of the true elaborated sap is downward, it scarce admits of a 

 doubt that the progress of the growth is also downward, from 

 the leaves to the roots. And on no other supposition can we 

 account for such facts as the following. 



478. Girdle an ere wnous tree by removing an entire ring of its bark. It will nourish 

 still during one growing season, and form a new layer of wood and bark everywhere 

 above tne wound, as before, but not at all below. The next season the tree will dio 

 Why ? Because the true sap returning cannot descend to nourish the roots. But in a 

 few cases trees are sa?d to have survived this process. In such cases the medullary rays 

 may have completed tl*c broken currents. On arriving at the ring, the descending sap 

 flows inwardly by the Medullary rays, making a detour, and appears again in the bark 

 below the interruption. (See Class Book, p. 155, for a further illustration of this subject.) 



Revieiv. 468. Direction of the flowing sap. 409. How it advances in the tissue of a 

 Cryptogam. How in th \ jiigher plants. 470. Vessels for air only. 471. Tendency of the 

 "jw. By what tissues? By which layers, and why ? 472. The crude sap. 473. Account 



r the overflow For itb cessation. 474. The change to true sap. Trace its return from 



for the overflow For itb cessation. 474. The change to true sap. Trace its return from 

 the leaves. 476. Specify th 

 downward? 47b. A proof. 



the leaves. 476. Specify Uio places of deposit. 477. Does grow in. progress upward or 

 . A pr 



