CHAPTER IV. 



STRUCTURE AND FUNCTIONS OF THE ROOTS. 



100. THE Roots are the parts of the plant, on which it is 

 chiefly dependent for the supply of the moisture, which its growth 

 requires ; and they also serve to fix it in the earth. That they 

 absorb or suck up fluid with great rapidity, may be easily shown 

 in the following manner. Take any small plant that is growing 

 in the soil, and immerse its roots (first clearing them of earth) in 

 a tumbler of water. If the plant be exposed to the light of day, 

 and especially if the sun shine brightly upon it, the water will 

 disappear very much faster from the glass, than from one expos- 

 ing the same surface, and placed in the same circumstances, but 

 without the plant. And if the specimen continue to grow and 

 flourish, it will take up many times its own weight of water in 

 a short period. Thus, four plants of Spearmint, grown during 

 56 days with their roots in water, and themselves weighing all 

 together but 403 grains, have been observed to take up 54,000 

 grains, or about seven pints of the fluid. 



101. Of the water thus absorbed, a small proportion only is 

 retained within the plant, serving as a part of its food. The 

 greatest part of it is sent off again from the leaves, by a process 

 hereafter to be described (Chap, vm.), termed Exhalation ; and 

 the rapidity of Absorption is in part governed by the rapidity 

 of Exhalation. The latter is nearly checked by the absence of 

 light ; and, accordingly, plants are found to absorb but little in 

 the night, or in a dark room. Any of the causes, which will be 

 subsequently stated to influence the latter, affect the former in a 

 nearly corresponding proportion. The object of the introduction 

 of a quantity of fluid into the vegetable system, so much larger 



