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 crcwnous tree by removing an entire ring of its bark. It will flourish 

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

 anove tne wound, at. before, but not at all below. The next season the tree will din 

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

 few cases trees are said to have survived this process. In such cases the medullary rayi 

 may have completed tlc broken currents. On arriving at the ring, the descending cap 

 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.} 



Review. 468. Direction of the flowing sap. 469. How it advances in the tissue of a 

 Cryptogam. How in th/> aigher plants. 470. Vessels for air only. 471. Tendency of the 

 flow. By wbat tissues T By which layers, and why ? 472. The crude sap. 473. Account 

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

 the leaves. 476. Specify t!io placed of deposit. 477. Does growih progress upward or 

 downward ? 47b. A proof. 



