ROOTS ABSORB AQUEOUS SOLUTIONS, AND OXYGEN. 77 



Known and understood, the precise way in which these ends are accom- 

 plished by the roots, and the powers with which they are invested, are 

 still to a considerable degree matters of dispute. 



I. It appears certain that they are possessed of the power of absorb- 

 ing water in large quantity from the soil, and of transmitting it upwards 

 to the stem. The amount of water thus absorbed depends greatly upon 

 the nature of the soil and of the climate in which a plant grows, but 

 much also upon the specific structure of its leaves and the extent of its 

 foliage. 



II. The analogy of the leaves and young twigs would lead uS to 

 suppose that, when in a proper state of moisture, the roots should 

 also be capable of absorbing gaseous substances from the air which 

 pervades the soil. Experiment, however, has not yet shown this to be 

 the case. 



We know, however, that they are capable of absorbing gases through 

 the medium of water. For if the roots of a plant are placed in water 

 containing carbonic acid in the state of solution, this gas is found gradu- 

 ally to disappear. It is extracted from the water by the roots. And if 

 the water in which the roots are immersed be contained in a bottle only 

 partially filled with the liquid, while the remainder is occupied by at- 

 mospheric air, the oxygen in this air will also slowly diminish. It will 

 be absorbed by the roots through the medium of the water.* 



Again, if in the place of the atmospheric air in this bottle, carbonic 

 acid be substituted, the plant will droop and in a few days will die. The 

 same will take place, if instead of common air or carbonic acid, nitro- 

 gen or hydrogen gases be introduced into the bottle. The plant will not 

 live when its roots are exposed to the sole action of any of the three. 



It is obvious, therefore, that the roots of plants absorb gaseous sub- 

 stances from the air which surrounds their roots, at least indirectly and 

 through the medium of water. It appears also that from this air they 

 have the power of selecting a certain portion of oxygen when this gas is 

 present in it. Thirdly, that though they can absorb carbonic acid to a 

 limited amount without injury to the plant, yet that a copious supply of 

 this gas, unmixed with oxygen, is fatal to vegetable life. This deduction 

 is confirmed by the fact that, in localities where carbonic acid ascends 

 through fissures in the subjacent rocks and saturates the soil, the growth, 

 of grass is found to be very much retarded. And, lastly, since nitrogen 

 is believed not to be in itself noxious to vegetable life, the death of the 

 plant in water surrounded by this gas, is supi)Osed to imply that the pre- 

 sence of oxygen is necessary about the roots of a growing and healthy 

 plant, and that one of the special functions of the roots is constantly to 

 absorb this oxygen. 



This supposition is in accordance witli the fact that, in the dark, the 

 leaves of plants absorb oxygen from the atmosphere ; for we have al- 

 ready seen reason to expect that, from their analogous structure, the roots 

 and leaves in similar circumstances should perform also analogous func- 

 tions. At the same time, if the roots do require the access and presence 



' It will be recollected that water absorbs about 4 per cent, of its bulk of air from the at- 

 mosphere, of which about one-third is oxygen. If the roots extract this oxygen from the 

 water, the latter will again drink in a fresh portion from the atmospheric air which floats 

 above it. 



4* 



