THE STEM 49 
towards the outside, as in the case of roots. The 
CAMBIUM is a tender meristematic tissue separating the 
phloem from the xylem. The whole of these three 
tissues make up what is called a collateral open bundle, 
collateral because the phloem and xylem are placed side 
by side, open because of the presence of the cambium 
which enables further growth to take place. This cam- 
bium readily divides and produces new cells, phloem cells 
towards the outside and xylem cells towards the interior. 
Further cambium is formed out of some of the cells in 
vase bund pe I a 
Hee 
sk ch £4 Sp. ‘sp PP. 
Fic. 32.—Longitudinal Section through Stem of Helianthus tuber. 
cort. Cortex. ¢. Epidermis. p. Parenchyma of cortex. c. Collenchyma. 
b.s. Endodermis. f. Pericycle fibres. s.t. Sieve tube. cb. Cam- 
bium. d’d, Pitted vessels. s.p. Spiral vessels. pp. Pith. (From 
Darwin’s ‘‘ Elements of Botany ”.) 
the medullary rays and connects the cambium of one 
bundle with that of the next. Consequently in time we 
have a ring of cambium concentric with the endodermis, 
and, as this divides and produces new cells, we get in an 
older stem a continuous ring of xylem and a continuous 
ring of phloem. 
The new cells produced by the cambium are called 
SEcoNDARY PHLOEM and XyueEm, and the process is 
called SEconDARY THICKENING. It is due to the work 
of the cambium that tree trunks are found with a girth 
4 
