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, fe, and the cambium, 7c, 
formed between these wedges, continues to grow from its 
inner and from its outer surface, and thus causes a permanent 
SAX 
g 
1 
Fia. 53. — 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; 2, earliest wood formed; MM, 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 
1 See Gregory’s Plant Anatomy, Chapter VII. 
