152 ABSORPTION. 



CHAPTER VI. 



§ 1. ABSORPTION. 



770. Office op the boot. The absorption of liquids, containing in 

 solution the food of the plant, is the peculiar and indispensable ofBce of 

 the root, as may be shown by an 



771. Experiment. Take a small growing plant from the earth and immerse it by 

 its roots, which should be nearly or quite entire, in a cup containing a definite quan- 

 tity of water. Place near it another cup with a like quantity of water to indicate 

 the amount of evaporation. The difference of the diminution in the two cups will 

 be the amount of absorption. A plant of spearmint has thus been found to absorb 

 more than twice its own weight per day. Every one is famiUar with the rapid dis- 

 appearance of water from the roots of potted plants, as Hydrangea, Oleander. 



772. The absorbents. An impervious epidermis destitute of sto- 

 mata everywhere clothes the roots, excepting its fibrillse and the tender 

 extremities of the rootlets. No part, therefore, is capable of absorp- 

 tion except the latter. But these, by their multiplied numbers, present 

 an adequate absorbing surface to the soil. 



773. Experiment. Let a growing radish be placed in such a position that only 

 the fibrils at the end may be immersed in water ; — it will continue to flourish. But 

 if the root be so bent that the fibrils shall remain dry while the body of the root 

 only is immersed, the plant will soon wither, but will again revive if the fibrils be 

 again immersed. 



774. Inference. Hence, in transplanting a tree almost the only 

 danger to its life arises from the difficulty of preserving a sufficient 

 number of these rootlets. 



775. The force with which plants absorb fluids by their roots 

 is surprisingly great, as shown by 



776. Experiment. If the stem of a grape-vine be cut off when the sap is as- 

 cending, 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 mercu- 

 rial gauge to a vine thus severed, and found the upward pressure of the sap equal 

 to twenty-six inches of mercury, or thirteen pounds to the square inch. 



777. But what causes this absorption of fluids in a direction con- 

 trary to gravitation ? In explanation of this phenomenon reference 

 has been made to two well-known principles in physics, viz., to capillary 

 attraction by the tubular vessels and to endosmose by the closed cells, 

 which are far more numerous. 



778. Experiment. Invert the end of several open thermometer tubes in a colored 

 Kquid. It will be seen rising in the tubes above its level, to various heights — high- 

 est in the smallest calibre. 



779. Exp. Suspend a napkin in such fashion that its lowest comer shall dip 

 into a cup of water. In a few hours the water will have ascended into the napkin . 

 These are results of capillary attraction. 



