254 THE SECOND BOOK OF BOTANY. 



ringing of fruit-trees is one of the means of increas- 

 ing the product of fruit above the ring. The forma- 

 tion of potatoes may be prevented by ringing the 

 cortical layers of the stem. Tie a band tightly 

 around the bark of a young branch. After a little 

 time the branch swells, and forms a cushion above 

 the ligature, while, below, it preserves its former 

 size. When bark is accidentally rubbed off, the new 

 growth, by which the place is gradually covered, 

 comes from above. In monocotyledons, the elabo- 

 rated sap descends along the fibres of the liber of 

 each of the woody bundles, and in this way furnishes 

 the cambium with nourishing materials. 



In brief, then, water, containing the dissolved 

 food of plants, is absorbed by the extremities of the 

 roots. It rises through the latest-formed wood to 

 the cellular tissue of the leaves, and is there submit- 

 ted to the action of air and light. Changed to elab- 

 orated sap, it descends by the inner layers of the 

 bark, yielding up, in its course, nutritious material to 

 nourish all parts of the plant, till it reaches the root, 

 from which it started. 



Such is the course of the circulation in spring, 

 when the leaves are young and active. Later in the 

 season, as the woody tissues are more hardened, the 

 sap rises in the cellular tissue. In autumn, the leaves 

 are obstructed by the deposits of mineral matter, so 

 that sap cannot flow in them ; they dry up, and fall, 

 evaporation ceases, and, with it, the movement of the 

 sap. The so-called spongioles, however, continue to 

 act, and so the tree is gorged with liquid before the 

 winter sets in. This liquid dissolves the various pe- 

 culiar matters deposited in the cells of the plant, and 



