TRANSPIRATION AND RESPIRATION. 149 



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 crcerenous 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 tua 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 eaJd to have survived this process. In such cases the medullary rays 

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

 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.) 



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

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

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

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

 the leaves. 476 Specify Caj places of deposit. 477. Does growth progress upward or 

 downward ? 47&. A prooL 



CHAPTER X. 



TRANSPIRATION AND RESPIRATION. 



479. Transpiration relates to that important office per- 

 formed by the leaves and other green organs, whereby pure 

 water is separated from the crude sap and given off into the air. 

 It takes place chiefly through the stomata, and is greatest by 

 day, and in a warm, dry atmosphere. 



480. Upon the activity of transpiration depends also the 

 amount of absorption. It not only makes room for the fluids 

 from below to enter, but by disturbing their equilibrium it 

 creates an upward tendency, as the flame of a lamp draws the 

 fluid up the wick. All the mineral and organic constituents of 

 the sap are of course left behind, in the plant. 



