220 A TEXTBOOK OF BOTANY [Cn. V, 3 



from the root. In cross sections one can see the nbro-vascular 

 bundles lying so closely towards the center as greatly to re- 

 strict the area of the pith, or even to obliterate it altogether, 

 though there is always a relatively thick cortex (Fig. 163). 



The tips of the soil roots of different plants are far more 

 uniform in size, and especially in diameter, than are the 

 leaves and young stems, of course because of the more 

 uniform environment presented by the soil. Exact measure- 

 ments show that in ordinary plants, the roots at the growth 

 zone vary in diameter from .3 to 1.07 mm. with a mean at 

 .67 mm., while the side roots vary from .19 to .79 with a mean 

 at .53, giving a conventional constant of .6 mm. for the diam- 

 eter of root tips in general. This size bears without doubt a 

 relation to the conditions of water absorption by the roots, 

 analogous to the relation of leaf-thickness to light (page 

 33), though the precise factors have not yet been deter- 

 mined. 



Backwards the young white tips merge gradually into the 

 familiar brown, fibrous roots. Cross sections thereof show 

 the formation of a corky bark, the beginning of a secondary 

 growth in thickness of the bundles (in exogenous kinds), 

 and a general aspect of toughening of the tissues; for this 

 is the part of the root which seems to take much of the strain 

 of the anchorage function. 



Jnjherbaceous plants, as a rule, the roots remain fibrous, 

 but in shrubs and trees they grow continuously in thickness 

 by the activity of the cambium, quite after the manner of 

 the stem. Thus they develop a distinct bark and wood, 

 with annual rings, medullary rays, and other features already 

 familiar in stems. Indeed, except for their underground 

 position, such roots are practically stems. 



3. THE CELLULAR ANATOMY OF ROOTS 



As with other plant organs, the cellular anatomy of roots 

 is linked so closely with their functions that the two recipro- 

 cally throw light upon one another. 



