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 



FIG. 69. Diagram to illustrate Secondary Growth in a Dicotyledonous Stem. 



7?, 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 ; M, 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 



