CIRCULATION. 147 



4C6. Other means of absorption. The office of absorp- 

 tion is not performed by the root alone. Every green part, but 

 especially the leaf, is capable of absorbing gases and watery vapor. 



Every one knows how greatly plants, when parched and withered by drought, are re- 

 vived by a shower which sprinkles their leaves without reaching their roots. Air-plants, 

 or epiphytes ( 208), such as the Long-moss and Epidendrum, must rely on this source 

 chiefly for the supply of their food; and when the dissevered stems of such plants af tl 

 Houseleek grow without roots, suspended by a thread in air, it is evident that all the t 

 nourishment comes through their leaves. 



467. The use of absorption in the vegetable economy is not 

 merely the introduction of so much water into the plant, but to 

 obtain for its growth the elements of its food held in solution, 

 whether gaseous or earthy. In attaining this object, the roots 

 seem to be endowed with a certain power of selection or choice, 

 which we cannot explain. Thus, if wheat be grown in the same 

 Boil with the pea, the former will select the silica along with the 

 water which it absorbs, in preference to the lime; the pea selects 

 the lime in preference to the silica. Buckwheat will take chiefly 

 magnesia; cabbage and beans, potash. This fact shows the 

 importance of the rotation of crops in agriculture. 



Review. 460. Principal office of the Root. Illustrate by a young plant of Spearmint 

 By Hydrangea. 461. What special organs are the absorbents? What care in transplant- 

 ing trees ? 462. How prove the great force of absorption ? 463. Causes of the ascent of 

 the sap. Illustrate capillary attraction. 465. Illustrate Endosmose. What other explana- 

 tion in 471, and 480? 466. What other organs may absorb? Several illustrations. How 

 are Tree-mosses nourished? 467. Uses of absorption to vegetation. Have the roots the 

 power of choice ? Give examples. 



CHAPTER X. 



CIRCULATION. 



468. The fluids which are thus taken into the system by ab- 

 sorption cannot remain inactive and stagnant. As their inward 

 flow is regular and constant in its season, so must be their up- 

 ward and outward flow, in a course more or less direct, toward 

 the parts where they find an outlet or a permanent fixture. 



469. In those Cryptogams which are composed of cellular 

 tissue alone, the circulation of the sap consists only of a uniform 

 diffusion from cell to cell throughout the mass, as through a 

 sponge. In the higher plants, the different tissues perform ap 



