CHAPTER II 

 THE STRUCTURE OF THE ROOT 



§ 3. Wliat is the structure of the absorptive organ, the root ? 



We are first of all interested in the general structure of the 

 root. This organ represents the direct downward continuation 

 of the stem, and presents, like the latter, a central cylinder 

 (Fig. I G), which is surrounded by a softer tissue, the cortex 

 (Fig. I B). As the woody cylinder of the uninjured root 

 never reaches the periphery, but is always surrounded by 

 cortical tissue, it is necessarily the latter which is especially 

 concerned in the absorption of nutritive substances from the 

 soil. The central woody tissue, in which the naked eye can 

 often distinguish large wide vessels {&), has the function of 

 conducting the dissolved substances to all parts where they 

 may be required. 



(a.) The Absorptive Portion of the Boot. 



If we take a healthy plant carefully from loose soil, so as 

 not to injure the delicate ramifications of the root, and if we 

 wash the latter under a moderately strong jet of water, it will 

 be possible to free the roots almost entirely from the adhering 

 particles of soil. Only one portion of each rootlet will retain 

 its covering of grains of sand or particles of earth, and these 

 will seem to be stuck on to the root. With the aid of a 

 microscope we observe that delicate contorted root-hairs (Fig. 

 I S) have in reality fixed themselves to these small particles 

 of the soil and hold them very tightly. That region of the 

 root which is provided with this covering of sand is the portion 

 which is able to take up water from the soil. The absorptive 

 region of the root is therefore restricted to a certain youth- 

 ful zone, which begins close behind the smooth, often trans- 

 parent root-tip and reaches back about an inch. The older. 



