132 SECONDARY THICKENING OF ROOTS 



The root soon comes to possess a broad ring of secondary 

 wood and phloem (Fig. 63, C), similar to that of the stem, and, as 

 in the latter, traversed by primary (P.r.) and secondary rays. 

 The former, which, especially in herbs, often attain a considerable 

 width, are situated along the same radii as, and are equal in 

 number to, the primary xylem-groups (P.xy.). When these pri- 

 mary rays are broad and consist only of parenchyma, separate 

 wedges of secondary vascular tissues result. The radial extension 

 of the parenchymatous rays is not so marked as that of the 

 wood, so that certain diarch roots, after secondary thickening, 

 sometimes exhibit a band-like structure (e.g. Nettle). The 

 secondary rays, like those of the stem, are generally narrow. 



The vessels and wood-parenchyma of the secondary wood of 

 the root are relatively more numerous and more evenly dis- 

 tributed than in the stem, so that the annual rings are usually 

 less conspicuous. In its detailed structure the secondary phloem 

 is similar to that of the stem, and, except for the points already 

 mentioned, the same is true of the secondary wood. In the 

 root, as in the aerial axis, the production of secondary tissues 

 forces the primary phloem progressively farther from the centre. 

 Old, secondarily thickened roots resemble stems very closely, 

 but, when the central tissues are preserved, the original root- 

 structure can be traced by following down the primary rays 

 and locating the protoxylem-groups at the periphery of the 

 metaxylem. 



The storage of food in fleshy roots is effected by thin-walled 

 parenchyma, which is often copiously developed both in the 

 secondary phloem and in the secondary xylem. The vessels of 

 the latter then form isolated groups, which are either scattered 

 or arranged in radial files (e.g. Salsify, Tragopogon, Fig. 78, B) 

 in the storage tissues, a distribution facilitating rapid transference 

 from the storage cells when growth is resumed. In such roots 

 the limits of secondary xylem and phloem are often difficult to 

 distinguish, unless the cambium be first located. 



The process of secondary thickening so far described is that 

 normally found in the vast majority of Dicotyledons and Conifers, 

 but a few so-called anomalous types merit a brief consideration. 



