STRUCTURE 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 Fig. 53. The cambium 

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

 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 Dieotyledonons Stem. 

 i?, the first-formed bark ; j>, 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; 5, groups of bast-cells; /A, wood of tlie original wedges; ifh^ wood 

 formed between "wedges ; x, earliest wood 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 Anatomy, Chapter Vil. 



