62 ELEMENTS OF BOTANY. 
In C, the section shows a considerable advance in growth: 
the fibro-vascular bundles are larger and are now connected 
by a rapidly growing layer of tissue, cb. 
As growth continues, this layer becomes the cambium layer, 
composed of thin-walled and rapidly dividing cells, as shown 
in Fig. 41. ; 
85. Secondary Growth. — From the inside of the cambium 
layer the wood-cells and ducts of the mature stem are pro- 
duced, while from its outer circumference the new layers of 
the bark proceed. From this mode of increase, the stems of 
dicotyledonous plants are called exogenous, that is, outside- 
growing. The presence of the cambium layer on the outside 
of the wood in early spring is a fact well known to the school- 
boy who pounds the cylinder cut from an elder, willow, or 
hickory branch until the bark will slp off and so enable him 
to make a whistle. The sweet taste of this pulpy layer, as 
found in the white pine, the slippery elm, and the basswood, 
is a familiar evidence of the nourishment which the cambium 
layer contains. 
With the increase of the fibro-vascular bundles of the wood 
the space between them, which appears relatively large in 
Fig. 52, becomes less and less, and the pith, which at first ex- 
tended freely out toward the circumference of the stem, becomes 
compressed into thin plates so as to form medullary rays. 
These are, as already stated, of use in storing the food 
which the plant in cold and temperate climates lays up in 
the summer and fall for use in the following spring, and in 
the very young stem they serve as an important channel for 
the transference of fluids across the stem from bark to pith, 
or in the reverse direction. On account, perhaps, of their 
importance to the plants, the cells of the medullary rays are 
among the longest-lived of all vegetable cells, retaining their 
vitality in the beech tree sometimes, it is said, for more than 
a hundred years. 
