5z8 



THE STRUCTURE OF ECONOMIC PLANTS 



par 



The slightly elongated cells of the inner phloem are rich in 

 cell contents, and the behavior of the cytoplasm in fixed tissue 

 suggests that there are cytoplasmic connections between adjacent 

 protophloem elements in the same vertical series, although actual 

 perforations cannot be detected. Following the development of 



the inner protophloem, 

 further differentiation is 

 centrifugal and sieve 

 tubes occur in the later- 

 formed metaphloem 

 groups. Somewhat later, 

 sieve tubes also develop 

 in the outer phloem 

 groups. These are simi- 

 lar to those of the inner 

 pel fib (0) phloem, but they are laid 

 down centripetally in- 

 stead of centrifugally. 



Coincident with the 

 progressive development 

 of the phloem groups, 

 the protoxylem becomes 

 more extensive and the 

 elements formed have 

 closely spaced rings and 

 flat spirals. Continued 

 development results in 

 the formation of larger 

 metaxylem elements with 

 scalariform or scalari- 

 form-reticulate walls. 

 As the primary xylem is 

 differentiating, the inner phloem groups increase in size and the 

 outer ones become larger and more distinct. 



While the maturation of the primary tissues of the vascular 

 bundles is still taking place, there is an activation of the fas- 

 cicular cambium which forms a distinct region prior to the dif- 

 ferentiation of well-defined sieve tubes in the inner phloem. 

 Subsequent divisions of the cambial initials result in the formation 

 of two or three layers of cells between the primary xylem and the 



-ph (0) 



-xy 2 



-ph(i) 



^-—xy 1 



- -pel fib (i) 



- — pi 



Fig. 2.80. Transection of sector of mature stem : 

 ca, cambium ; col, collenchyma ; en, endodermis ; ep, 

 epidermis; par, parenchyma; pel fib (i), inner pericyclic 

 fibers ; pel fib (0), outer pericyclic fibers ; ph (/), inner 

 phloem; ph (0), outer phloem; pi, pith; xy i, primary 

 xylem ; xy i, secondary xylem. 



