96 FOUNDATIONS OF BOTANY 
one-year-old Aristolochia-stem (Fig. 56) shows a decided 
similarity between the two. In both cases we have the 
central pith, the regularly grouped bundles, and cambium 
(or in Fig. 68, C, a tissue which will grow into cambium), 
— part of it in the bundles and part of it between them. 
In the young monocotyledonous stem the grouping of 
the bundles is less regular than that just explained. This 
is shown by Fig. 52. A much more important difference 
consists in the fact that the monocotyledonous stem has 
usually no permanent living cambium ring. Annual dicoty- 
ledons, however, are also destitute of permanent cambium. 
108. Secondary Growth. — From the inside of the cam- 
bium layer the wood-cells and ducts of the mature stem 
are produced, while from its outer circumference proceed 
the new layers of the inner bark, composed largely of sieve- 
cells and hard bast. From this mode of increase the stems 
of dicotyledonous plants are called exogenous, that is, out- 
side-growing. The presence of the cambium layer on the 
outside of the wood in early spring is a fact well known 
to the schoolboy, who pounds the cylinder cut from an 
elder, willow, or hickory branch until the bark will slip © 
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. 68, becomes less and less, and the pith, which 
at first extended freely out toward the circumference of 
the stem, is at length only represented by thin plates, the 
medullary rays. 
