STRUCTURE OF THE STEM 97 
These are 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 transfer- 
ence of fluids across the stem from bark to pith, or in the 
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Fie. 69. — 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 ; 5b, groups of bast-cells; fh, wood of the original wedges ; 
ifh, wood formed between wedges; z, earliest wood formed; WM, pith. 
reverse direction. On account, perhaps, of their impor- 
tance to the plants, the cells of the medullary rays are 
among the longest lived of all plant-cells, retaining 
their vitality in the beech tree sometimes, it is said, for 
more than a hundred years. 
After the interspaces between the first fibro-vascular 
bundles have become filled up with wood, the subsequent 
