72 ESSENTIALS OF BOTANY 
outside of the wood in early spring is a fact well known 
to the schoolboy who pounds the cylinder cut from an 
alder, 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 
Fie. 44. Diagram to illustrate Secondary 
Growth in a Dicotyledonous Stem. 
R, the first-formed bark; p, mass of sieve-cells; 
ifp, mass of sieve-cells between the original 
wedges of wood; fc, cambium of wedges of 
wood; ic, cambium between wedges; b, groups 
of bast-cells; fh, wood of the original wedges ; 
ifh, wood formed between wedges; x, earliest 
wood formed; ¥, pith. 
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 ap- 
pears relatively 
large in Fig. 40, be- 
comes less and less, 
and the pith, which 
at first extended 
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 
