i62 THE LIFE OF THE PLANT 



differentiated from the wood ; in spring when the plant 

 is full of sap it even peels off easily. Non-botanists 

 presume, and formerly even botanists also presumed, 

 that there is a space in between the rind and the 

 wood, which is filled, especially in spring, with a thick 

 liquid, out of which the new parts of the plant can 

 be formed. Exact microscopic investigation has proved 

 that there is no such space, but at that part of 

 the stem there is a ring of exceedingly delicate juicy 

 tissue capable of continually forming new cells — 

 hence its name oi formative tissue, otherwise cambium. 

 In fig. 45, II, the cambium is shown as a dark ring 

 which cuts across the vascular bundles as well as the 

 medullary rays, the whole stem being divided by it 

 into two parts — the wood lying inside the ring, and the 

 rind lying outside it. Owing to the presence of this 

 continuous circular formative layer, which is absent 

 from the monocotyledons because their vascular bundles 

 are scattered instead of being distributed in a regular 

 circle, the stems of dicotyledons and conifers are capable 

 of long continued growth in thickness. Every year 

 this formative tissue deposits new rows of cells towards 

 both the wood and the bark ; only the wood increases 

 more rapidly (fig. 45, III.), and the rows of its cells are 

 deposited more evenly ; this is why it presents the 

 regular alternation of annual rings that we notice on 

 every transverse section of a tree. Let us now see 

 what is the anatomical structure of these two kinds of 

 vascular tissue on either side of the formative tissue, 

 i.e. in the wood and in the rind. On the wood side 

 we find almost exclusively fibres called wood fibres 

 (fig. 42, 6) and various vessels, pitted, reticulate, 

 spiral, and so on, but no sieve-tubes. On the bark side 

 we find very elongated fibres with very thick walls (fig. 

 42, 5) , similar to the fibres of the plants used for spinning 

 that we mentioned before, and the only tubes we meet 

 are the sieve- tubes mentioned above (fig. 43, II.). The 



