ABSORPTION. 67 



its posture. But its peculiar and most important function is 

 ABSORPTION, or drawing from the soil that food and moisture 

 which its growth absolutely requires. 



a. Let any small growing plant be taken from the earth, and immersed by its 

 roots in a glass of water. If it be then exposed to the light of day, or especially 

 to the sun, the water will disappear from the glass more rapidly than could be 

 expected from evaporation alone. A plant of spearmint has thus been found to 

 absorb water at the rate of more than twice its own weight per day. The water 

 thus absorbed by the roots is mostly sent off again, or exhaled through the leaves 

 (a process called EXHALATION), only a small part of it, together with the salts 

 which it held in solution, being retained for the use of the plant. 



155. The activity of absorption must, therefore, depend upon 

 the activity of exhalation ; and since the latter is dependent upon 

 the presence of light and heat, it follows that absorption will, in 

 general, be more active by day than by night. 



156. The root does not absorb moisture by its whole surface, 

 indiscriminately, but only by the spongioles at the extremities 

 of the fibrils, where the pores are not obstructed by the epider- 

 mis. From the spongioles it is conducted by the vasiform tissue 

 of the fibril to the vessels of the main root, and immediately 

 carried up the stem, and distributed to all parts of the plant. 



a. If a growing radish be placed in such a position that only the fibres at the 

 end may be immersed in water, the plant will continue to flourish. But if the 

 root be so bent that the fibrils shall be curved up to the leaves, and only the 

 curved body of the root be immersed, the plant will soon wither, but will soon be 

 again revived, if the fibres be relaxed and again submerged. 



b. Hence, in transplanting trees, too much care cannot be taken to preserve, 

 uninjured, as many as possible of these tender, absorbing fibres. 



157. The force with which- plants absorb fluids by their roots 

 is very great, as is proved by experiment 



a. If the stem of a vine be cut off when the sap is ascending, and a bladder be 

 tied to the end of the standing part, it will in a few days become distended with 

 sap, even to bursting. Dr. Hales contrived to fix a mercurial gauge to a vine thus 

 severed, and found the upward pressure of the sap equal to 26 inches of mercury, 

 or 13 Ibs. to the square inch. 



158. The causes of the absorption of fluids, by the roots, have been the subject 

 of much inquiry. It has generally been said to be due to capillary attraction ; 

 but, unfortunately for this theory, there are no capillary tubes in the vegetable 

 structure, but only closed cells, more or less elongated, through the membranous 

 walls of which the fluids must force their way. There is, however, a phenomenon 



