CIRCULATION. 147 



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, 

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

 soil 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. 



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

 By Hydrangea. 401. What special organs are the absorbents'? What care in transplant- 

 ing trees ? 402. How prove the great force of absorption ? 403. Causes of the ascent of 

 the sap. Illustrate capillary attraction. 405. Illustrate Endosmose. What other explana- 

 tion in 471, and 480 ? 406. 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 IX. 



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- 

 propriate offices in the circulation; some conducting upward, 

 some downward, some conveying the crude sap, some latex, and 

 some air. 



470. Spiral vessels and others of the trachenchyma are gen 

 orally filled with air, and take no part in the circulation of fluids, 

 except in the Spring, w r hen the whole system is gorged with sap. 

 The intercellular passages, also, generally circulate air alone. 



471. From the roots the newly absorbed fluid flows upward 

 through the stems and branches, toward the buds, leaves, and 

 flowers, being probably drawn thither into them by the exhala- 



