STRUCTURK OF ROOT 



41 



A longitudinal section through the root 

 tip of shepherd's purse, showing the 



central vascular axis (pi), surj-ounded 

 by the cortex (p), outside of tne cor- 

 tex the epidermis (e) which disap- 

 pears in the older parts of the root, 

 and the prominent root-cap (c). 



office of these cells is merely to gi\e the root strength and form, while 

 through them and between them the moisture absorbed by the outer 

 layer reaches the central cylinder within. 



The innermost layer of cells of 

 the curtex _forms a very complete 

 and very rigid cylinder, enclosing 

 the central cylindi'r. This endodermh 

 consists of regularly formed, closely- 

 fitting cells which prevent the escape 

 (if ])lant food on its course upward 

 through the central cylinder of older 

 roots. In younger plants, however, 

 the passage of moisture from the 

 surface to the cylinder is not 

 hindered. 



The pericycle, though not very 

 distinct in many roots, is the outer 

 cell layer of the central cylinder. 

 From single cells within it, arise all 

 secondary roots. By pushing their 

 way outward through the cortex and surface layer, and by repeated 

 cell divisions they soon elongate and become tributary feeders. This 

 internal origin of the branch roots can be readily seen by peeling off 

 the cortex, which lays liare the attachment. 



The central cylinder consists for the most part of tubes which are 

 of use in carrying the plant food upward into the stem and leaves. 



CONDITIONS AFFECTING ROOT GROWTH. The factors 



affecting root growth are the factors which affect the yield of the crop. 

 (i) In order that the younger and more tender rootlets may 

 push through the soil, its *structure must be quite fine. A root will 

 not cross a large interspace between lumps of earth. 



(2) Corn roots draw almost entirely upon the capillary water of 

 the soil. In case of extreme drought they may possibly use some of 

 the hygroscopic moisture. Very little, if any of the gravity water, that 

 which is drained from the soil in tiling, is utilized by the plant. 



(3) Roots avoid a cold soil and if the ground is of a low tem- 

 perature will feed near the surface. 



(4) The entrance of oxygen into the soil is necessary to insure 

 the spread of root growth. 



(5) Roots seek and require the presence of plant food in the soil. 



*TIie words structure and texture are often used synonymously. The structural peculiarities 

 are those wirich interest the geologist, the tcxtural belong more properly to the mineralogist. 

 But the usages of geologists differ in the employment of terms of this kind, and there can be no 

 precise limit drawn, separating structures from textures. (Century Encyclopedia.) 



