CHAPTER XVII 

 ABSORPTION OF WATER 



Roots and Root Systems. — The root system of any species of plant is 

 often as distinctive in form and structure as its aerial portions. Each species 

 has, when growing in its usual or "normal" type of habitat, a characteristic 

 set of roots, just as it has a recognizably distinctive top when growing within 

 its usual range of climatic conditions. Root systems are subject to modifica- 

 tions as a result of the influence of various soil factors, just as the form, height, 

 spread or other features of the tops may be modified in accordance with the 

 climatic condition to which they are subjected. A coniferous tree may produce 

 a stately, cone-shaped crown if growing in a favorable habitat, while another 

 individual of the same species will be only a straggling, scrawny shrub if 

 located near the timber line on a mountain. Similarly, a plant may develop 

 a deep, profusely branched root system in a well-drained soil, while another 

 individual of the same species will produce a shallow root system of entirely 

 different aspect in a soil which is water-logged to within a foot or two of its 

 surface. 



Superficially roots resemble stems, inasmuch as both are usually elongate, 

 more or less cylindrical structures. There are, however, a number of im- 

 portant distinctions between the two types of organs. Roots are generally 

 much more irregular in shape than stems. They are not differentiated into 

 distinct nodes and internodes, and hence the branching of roots follows a 

 much less regular pattern than the branching or bearing of lateral organs 

 by stems. With few exceptions the growing tip of every root is capped with 

 a distinctive zone of cells known as the root cap; such a tissue is absent from 

 the stem tips. The origin of lateral roots is very different from that of lateral 

 stems; the former develop from deep-seated meristems; the latter from peri- 

 pheral meristems. The arrangement of the primary tissues in roots is usually 

 different from the arrangement of the primary tissues of stems. Roots bear 

 no appendages which are comparable to leaves or flowers, and lack stomates 

 which are often present in stems. 



When a seed germinates the first root which appears is called the primary 

 root. This develops from an apical growing region which is already differ- 



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