MEDICAGO SATIVA 



32-7 



the first year which consists of a few scattered strands produced 

 early in the year and a larger group of fibers formed later in the 

 same season. 



The cortical and epidermal tissues do not persist for any great 

 length of time, since the cells of these zones are ruptured and 

 disintegrated by secondary activity. For this reason, the outer- 

 most portion of the mature root consists of a pericyclic region 



pd 

 pel 



-fib 

 ■ph2 



ca 



ra 



xy 2 



xy par 



— xy fib 



xy 1 



Fig. 165. Sectors of four-year-old tap root: at left, showing the outer portion; and at 

 right, the inner portion of root. Each annual addition in growth may be distinguished in 

 both xylem and phloem. In xylem, the spring wood has relatively large vessels and abundant 

 fibers. The late autumn wood has few or no fibers and small vessels surrounded by xylem 

 parenchyma. The end of each annual cycle is marked by an abrupt transition from small to 

 large vessels. In phloem, a bundle of fibers is laid down in spring, when the xylem is also 

 producing fibers. Sieve tubes and parenchyma are produced in late summer and fall, only to 

 be crushed as soon as the next fibers are produced in spring : ca, cambium ; fih, phloem fibers ; 

 ;)ir/, pericycle ; pd, periderm ; />A i, secondary phloem ; r^, xylem ray; xjy i, primary xylem ; 

 xy -L, secondary xylem; xy fib, xylem fibers; xy par, xylem parenchyma. (After Jones, Jour. 

 Agr. Km.) 



