STETJCTUEE OF THE STEM. 



63 



After the inter-spaces between the first fibro- vascular bundles 

 have become filled up with wood, the subsequent growth must 

 take place in the manner shown in Pig. 53. The cambium 

 of the original wedges of wood, fc, and the cambium, ic, 

 formed between these wedges, continues to grow from its 

 inner and from its outer surface, and thus causes a permanent 



Fig. 53. — Diagram to illustrate Secondary Growth in a Dicotyledonous Stem. 

 iZ, the first-formed bark ; ^, mass of sieve-cells ; ifp, mass of sieve-cells between the 

 original wedges of wood ; /c, cambium of wedges of Wood ; ic, cambium between 

 wedges ; ft, groups of bast-cells ; fh, wood of the original wedges ; i/A, wood 

 formed between wedges j x, earliest weod formed ; M, pith. 



increase in the diameter of the stem and a thickening of the 

 bark, which, however, usually soon begins to peel off from 

 the outside and thus soon attains a pretty constant thickness.' 

 86. Grafting. ■ — ■ When the cambium layer of any vigor- 

 ously growing stem is brought in contact with this layer in 



^ See Gregory's Plant Aiiat&my, Chapter VH. 



