THE STEM 125 

Phloem parenchyma. The phloém contains parenchyma cells 
whose structure is very similar to that of other parenchyma cells. 
These are known as phloém parenchyma. 
Cambium. While the other cells of the stem which are formed 
at the tip are being differentiated into permanent tissue, a 
single layer between the | 
xylem and phloém re- 
mains undifferentiated 
G igs. a 116). This Wisieseess: Be oe 
yer of meristematic WT ye Be 
; Ly er 
cells is known as the w f: 
cambium. The cam- 
bium consists of a sin- 
ole layer of cells which, 
by division, gives rise 
to xylem cells toward 
the center of the stem 
and phloém cells to- 
ward the periphery. 
The activity of the 
cambium cells thus en- 
larges the size of the 
bundles and, in conse- 
quence, that of the 
stem. This process is 
known as_ secondary 
thickening and will be 
discussed in detail in 
d, 
are’ 3 
H) HLA ciceeek oom \ ray 
A TK) 2.5, en bie) 

: Fic. 122. Longitudinal section of oil gland in 
a later section. orange peel. (x 450) 
At first the cambium 
is confined to the bundles, but later the parenchyma cells of the 
pith rays which lie between the edges of the cambium in the 
bundles divide and form a layer of cambium which reaches across 
the pith rays and connects that in the bundles, so that the cam- 
bium becomes a continuous cylinder (Fig. 139). The beginning 
of this process is shown in Fig. 113. 

