22 THE VITAL FUNCTIONS. 



ture, which they have hcen supposed to do by the stomata 

 interspersed on their surface. This, however, is not their 

 natural action; and they assume it only in forced situations, 

 when they procure no water by means of the roots, either 

 from having been deprived of these organs, or from their 

 being left totally dry. Thus, a branch separated from the 

 trunk, may be preserved from withering for a long time, if 

 the leaves be immersed in water: and when the soil has been 

 parched by a long drought, the drooping plants will be very 

 quickly revived by a shower of rain, or by artificial water- 

 ing, even before any moisture can be supposed to have pe- 

 netrated to the roots. 



It is by the extremities of the roots alone, or rather by 

 the spongioles which are there situated, that absorption takes 

 place: for the surface of the root, being covered in every 

 other part by a layer of epidermis, is incapable of perform- 

 ing this office. It was long ago remarked by Duhamel, that 

 trees exhaust the soil only in those parts which surround the 

 extremities of the roots: but the fact that absorption is ef- 

 fected only at those points has been placed beyond a doubt by 

 the direct experiments of Senncbier, who, taking two car- 

 rots of equal size, immersed in water the whole root of the 

 one, while only the extremity of the other was made to dip 

 into the water, and found that equal quantities were absorbed 

 in both cases; while, on immersing the whole surface of ano- 

 ther carrot in the fluid, with the exception of the extremity 

 of the root, which was raised so as to be above the surface, 

 no absorption whatever took place. Plants having' a yim- 

 form, or spindle-shaped root, such as the carrot and the 

 radish, are the best for these experiments. 



In the natural progress of growth, the roots are constant- 

 ly shooting forwards in the direction they have first taken, 

 whether horizontally, or vertically, or at any other inclina- 

 tion. Thus, they continually arrive at new portions of soil, 

 of which the nutritive matter has not yet been exhausted; 

 and as a constant relation is preserved between their lateral 

 extension and the horizontal spreading of the branches, the 



