
THE ROOT 69 
further back from the growing point. Secondary growth has 
been going on for some time. The greater part of the section 
shows secondary wood, the primary wood having been pushed 
towards the centre; the primary bast, still alternating with 
the primary wood, has been pushed towards the circumference 
by the formation of the secondary bast within it; whilst 
between the bast and wood lies the cambium. 
By the time this condition of things is reached the cortical 
parenchyma of the root has entirely disappeared, for the cells 
of the pericycle, which began to divide at the same time that 
the cambium was being formed, have developed a layer of 
cork, and the outer tissues have therefore become dried up. 
_ The following table may be found useful : 
COMPARISON OF ROOT AND STEM WHICH HAVE 
GROWN IN THICKNESS. 
ROOT. 
|. The primary groups of wood 
and bast lie along alternate 
radii. 
2. The protoxylem is the ex- 
ternal part of the primary 
wood. 
3. Opposite each group of pri- 
mary wood isawell-marked 
medullary ray, extending 
to the pericycle. 
4. The groupsof primary wood 
frequently unite across the 
centre, so that there is no 
pith. 
Longitudinal 
STEM. 
The primary groups of wood 
and bast lie along the same 
radii. ae 
The protoxylem is the internal 
part of the primary wood. 
The primary medullary rays 
are not so well marked. 
Except in a few cases—chiefly 
water plants—there is a 
well-marked pith. 
From what has been said it will be gathered 
Course of that the longitudinal course of bundles in the root 
Bundles. 
is simpler than it is in the stem. The bundles are 
more or less straight, extending from the growing point back- 
wards to where the root merges into the stem. From these 
bundles of the primary root other bundles passing into the 
lateral roots are given off. The course of bundles in the stem 
is more complicated, owing to the fact that leaves, the lateral 
outgrowths of the stem, are members unlike the stem, whereas 
