498 THE STRUCTURE OF ECONOMIC PLANTS 



peripheral portion of the cortex even in types where the ridges are 

 lacking. 



As the development of the axis proceeds, the endodermis and 

 cortical parenchyma fail to keep pace with the growth of the stele, 

 although there is evidence of some anticlinal division of the epider- 

 mal cells which become very much stretched tangentially. An 

 ephemeral periderm may originate in the cortical cells, but they 

 soon disintegrate; and, as this occurs, a phellogen is formed in the 

 pericycle which produces the periderm of the mature root. 



The Mature Fleshy Root. — In the mature fleshy root, the 

 epidermis, cortical parenchyma, and endodermis are no longer 



\-ca2 



Fig. 157. Transection of portion of cambial zone of larger fleshy root showing formation 

 of secondary cambium inside of group of secondary xylem elements : ca, cambium ; ca 2., 

 secondary cambium ; ^^ 2, secondary phloem; Ary i, secondary xylem. (After Artschwager, 

 Jour. Agr. Rej.) 



present and the protective "skin" consists of a pericyclic periderm 

 which has been maintained by an active phellogen throughout the 

 period of its growth and enlargement. Within the periderm, the 

 pericyclic zone is composed of large parenchymatous cells which 

 are filled with starch, and these abut projecting zones of phloem 

 consisting of radial rows of sieve tubes and companion cells which 

 are somewhat irregularly arranged because of the anomalous 

 method of axial growth. The large secondary sieve tubes have 

 transverse sieve plates, and in some regions two or three large 

 lateral sieve fields may be formed. The companion cells are smaller 

 in diameter than the sieve tubes but equal them in length. 



The cambium also produces vertical rows of lactiferous cells 

 which form the latex vessels. The cells retain their transverse 



