


58 ‘ FLOWERING PLANTS 
1. Wood-vessels—spiral, annular, pitted ; 
2. Wood-cells, or wood parenchyma ; 
3. Wood-fibres, or wood prosenchyma. 
Growth ine Ene stem increases in thickness by means of the 
Thickness. cambium ; this meristematic tissue forms bast 
and wood in the following way : 

Fria. 26.—DIAGRAM ILLUSTRATING FORMATION OF SECONDARY BAST AND 
Woop FRoM CAMBIUM. 
I, Asingle cambium cell. II, m, meristematic cell; 0, bast. III-V, 
m, meristematic cell ; b, bast; w, wood. 
Let I (Fig. 26) be a single cambium cell; this divides into 
two. There does not appear to be any fixed rule which of 
these two cells will next divide. If the inner—that is, the 
one nearest the wood—retains the power of dividing, then 
the outer forms bast (II, 0). This inner cell then divides into 
two cells, the outer of which will retain the power of dividing, 
(for the cells take it in turn to divide), and the inner becomes 
wood (III, w). The outermost cell next divides into two, the 
inner of which retains the power of dividing, the outer 
becoming bast. In this way secondary bast and secondary 
wood are formed. Sometimes, instead of forming wood and 
bast, the cambium gives rise to rows of thin-walled cells, 
which form the medullary rays. Cambium is especially 
active in the spring, but the formation of bast and wood 
goes on through the summer into the autumn. The wood 
formed in autumn is more compact than that formed in 
spring, and as the spring wood of one year comes next the 
