[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 305 



endogenous origin, especially the adventitious stems that develop from 

 roots. 



Lateral roots often appear mainly in two or more rows up and down 

 the root, because most of the root primordia originated opposite the 

 radially arranged ridges of the primary xylem. 



Some of the differences in the apical regions of stems and roots may 

 now be noted. In stem tips there is no structure comparable to the root 

 cap, even in the rhizomes that grow in the soil. The cells from the center 

 of the apical meristem are forerunners of pith which is surrounded by 

 the primary xylem, whereas in most roots thev are the forerunners of a 

 central xylem. At the root tip there are no lateral organs comparable to 

 the lateral primordia of axillary buds, foliage leaves, or the primordia of 

 flowers. Root hairs are present near the tips of roots of most plants. 

 They are usually short-lived, but in some species they may live for 

 months. 



The hormone known as auxin a appears to be necessary for cell en- 

 largement in both stems and roots, but as it is increased in amount cell 

 enlargement is decreased. The concentration of hormone necessary to de- 

 crease cell enlargement occurs naturally in roots, but usually not in 

 stems. These facts help us explain the difference in geotropism of stem 

 and root tips, as shown in Chapter XXIII. The direction of growth of 

 young lateral roots, like that of lateral stems, is influenced bv apical 

 dominance. The results of recent experiments indicate that hormones 

 necessary for growth in root tips may be made in stem tips, and vice 

 versa. 



Secondary growth of roots. Most of the root tissues mentioned above 

 are primary tissues that developed from the cells of the apical meristem. 

 Secondary growth in thickness results primarily from the development 

 of a vascular cambium as a cylindrical sheath between the xylem and 

 phloem. From the cells formed by this cambium, increase in xylem and 

 phloem occurs as in stems. 



The secondary growth may result in breaking and loss of epidermis. 

 The cortical tissue may continue enlarging for a time, or it too may be 

 broken and sloughed off. The pericycle survives in roots of some kinds 

 of plants much longer than in others. The thickened herbaceous roots of 

 some plants, such as those in long conical radishes and sweet potatoes, 

 are the result of growth of the vascular cambium plus further division 

 of the parenchyma cells of xylem, phloem, or pericycle. The concentric 

 rings seen in cross sections of beets are the cut edges of a series of cone- 



