20 THE VITAL FUNCTIONS. 



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 watering, even 

 before any moisture can be supposed to have 

 penetrated to the roots. 



It is by the extremities of the roots alone, or 

 rather by the spongioles which are there situ- 

 ated, that absorption takes place; for the surface 

 of the root, being covered in every other part by 

 a layer of epidermis, is incapable of performing 

 this office. It was long ago remarked by Du- 

 hamel, that trees exhaust the soil only in those 

 parts which surround the extremities of the 

 roots ; but the fact, that absorption is effected 

 only at those points, has been placed beyond a 

 doubt by the direct experiments of Sennebier, 

 who, taking two carrots 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 another carrot in the fluid, with 

 the exception of the extremity of the root, which 

 was raised so as to be above the surface, no ab- 



