Ch. V, 3] 



ANATOMY OF ROOTS 



223 



stand in-and-out from one another but alternately, or in 

 different radii. This arrangement, found in all roots, has 

 been viewed as adaptive, in removing the phloem out of the 

 path of transfer of the water from root hairs to ducts ; and 

 support is given this supposition by the fact that immedi- 

 ately behind the hair zone the arrangement is abandoned, 

 for the new xylem 

 and phloem made by 

 the developing cam- 

 bium stand in-and- 

 out from one another 

 as in stems. The 

 method by which 

 the cambium makes 

 the transition from the 

 one arrangement to 

 the other is easily un- 

 derstood by aid of 

 the figure. Endog- 

 enous roots do not, 

 of course, form a cam- 

 bium, but have sepa- 

 rated closed bundles 

 as in their stems. 



Just behind the hair 

 zone the cambium 

 begins the secondary 

 increase in thickness, 

 by addition of xylem 



from its inner and phloem from its outer face, precisely as with 

 stems. Farther back along the root, one can see here and 

 there in cross sections the mode of formation of the new side 

 roots, which come from the fibro-vascular bundles, and make 

 their way to the surface, as already described (Fig. 164). 



In the thick woody parts of the roots of shrubs and trees 

 the cellular anatomy is nowise essentially different from 



Fig. 164. — Longitudinal section of a root 

 of Corn, showing the origin of a side root ; 

 highly magnified. 



The side root develops in contact with a 

 fibro-vascular bundle, and "dissolves" its way 

 out, by action of enzymes, to the surface. 



