WOOD, CAMBIUM, AND BAST 21 
one seed-leaf) have bundles like the rest, but they have 
little or no “cambium,” that is to say, the cells very soon 
stop splitting up, and there are only the wood and bast 
sections left. The bundle is then said to be “closed,” 
and if the tree or plant is to grow another bundle has 
to be started in the “fundamental tissue” or mass of 
ordinary cells, such as we described in Chapter II. Now 
in the Dicotyledons the bundle remains “open,” that is 
to say, the cambium cells go on splitting and splitting 
so long as the plant continues to live. 
DIAGRAM OF CROSS SECTION OF YOUNG TWIG, 
I want you now to see how these bundles go to work 
in building up a big tree. 
Let us take the stem of a two-year-old seedling tree 
and cut a cross-section. We shall find a few bundles 
embedded in the flesh, say, four or five, each with the. 
woody part towards the centre, a narrow band of 
cambium cells, and then the bast part outside. But 
while the cambium is building up wood towards the 
centre, and bast towards the circumference, it is also 
forming cambium cells at its own free edges along the 
dotted line in the figure. These new cells, in their turn, 
busily construct wood on the inside and bast on the 
