STRUCTURE OF THE STEM 71 
87. The Early History of the Stem. — In the earliest 
stages of the growth of the stem it consists entirely of 
thin-walled and rapidly dividing cells. Soon, however, 
the various kinds of tissue which 
are found in the full-grown stem 
begin to appear. 
In Fig. 48 the process is shown 
as it occurs in the castor bean. 
At m, in B, is the central column 
of pith surrounded by eight fibro- 
vascular bundles, fv, each of 
which contains a number of ducts 
arranged in a pretty regular man- 
ner and surrounded by the fore- 
runners of the true wood-cells. 
In C the section shows a con- 
siderable advance in growth: the 
fibro-vascular bundles are larger 
and are now connected by a rap- 
idly growing layer of tissue, cb. 
As growth continues this layer 
becomes the cambium layer, com- 
posed of thin-walled and rapidly 
dividing cells, as shown in Fig. 45. 
88. Secondary Growth. — From 
the inside of the cambium layér 
the wood-cells and ducts of the 
mature stem are produced, while 
Fic. 43. Transverse Section 
through the Hypocotyl of 
the Castor-Oil Plant at Vari- 
ous Stages. (Considerably 
magnified. ) 
A, after the root has just ap- 
peared outside the testa of 
the seed; B, after the caulicle 
is nearly an inch long; C, at 
the end of germination; 7, 
cortex (undeveloped bark); 
m, pith; st, medullary rays; 
fv, fibro-vascular bundles; 
eb, layer of tissue which is to 
develop into cambium. 
from its outer circumference the new layers of the bark 
proceed. From this mode of increase the stems of dicoty- 
ledonous plants are called exogenous, that is, outside- 
growing. The presence of the cambium layer on the 
