32.6 THE STRUCTURE OF ECONOMIC PLANTS 



Secondary Thickening of the Root. — Secondary thickening 

 follows maturation of the primary tissues; and, in the main 

 roots which may become several years old, a massive woody axis 

 is formed. Cambial activity results in the formation of three, or 

 four, wedge-shaped sectors of secondary xylem which are separated 

 by broad parenchymatous rays that extend radially from each 

 protoxylem point toward the periphery. These main primary 

 rays are pericyclic in origin, but there are also broad secondary 

 xylem rays in each of the major sectors. (Fig. 165.) 



Jones (15) has described the annual rings formed in the develop- 

 ment of the tap root in connection with studies of winter injury, 

 and points out that they may be distinguished readily in most 

 instances. Exceptions occur in plants grown under dry farming 

 conditions, such as those of western Kansas, where the annual 

 increments of secondary growth are small and of uniform charac- 

 ter; and, also, in regions like southern California, where the 

 plants do not become dormant in the winter. The demarcation 

 of the rings is evident because the late summer and autumn 

 growth of xylem is characterized by vessels of small diameter and 

 small uniform parenchymatous cells w^ith relatively few or no 

 interspersed fibers, while the spring wood has larger vessels, less 

 parenchyma, and numerous fibers. The fibers are arranged in 

 groups separated by zones of vessels and tangential bands of 

 parenchyma, so that these layers may sometimes be mistaken for 

 annual rings. (Fig. 165.) In old roots, the enlargement and 

 division of the parenchymatous cells located near the center of 

 the axis result in the separation of the primary xylem elements 

 from the adjacent secondary xylem and from each other. 



The annual increments of secondary phloem are not as large as 

 those of xylem, but they are well defined. This is because the 

 first elements to be differentiated each growing season are phloem 

 fibers, while, later in the summer, fiber formation is followed by 

 the development of sieve tubes and parenchyma. With the produc- 

 tion of each succeeding group of phloem fibers, there is a tendency 

 for the sieve tubes and parenchyma of the preceding year to be 

 crushed; and although each annual increment of phloem consists 

 of a group of fibers, sieve tubes, and parenchyma, the last two 

 may be somewhat obliterated. Jones (15) has noted that the 

 secondary xylem of the first year has fewer fibers than occur in 

 succeeding years, and this is true also of the secondary phloem of 



