180 THE GROWTH OF ROOTS AND STEMS. 



altered (beyond growing in size and changing in shape) and 

 add to the living cells present in wood, bast, and rays. 



The shoot is continually growing and producing new leaves and 

 buds, so that year by year the number of leaves is becoming larger. 

 This involves a yearly increase in the weight of branches and leaves to 

 be supported and spread out to light and air. The branching of the 

 root means a continually growing area of absorbing surface, in the 

 form of root-hairs, in the soil, and since the root can make no organic 

 food to supply its increasing needs, more and more leaf -manufactured 

 food must be conveyed to it. 



All this means an increasing demand for more wood-tubes to conduct 

 water and salts from the roots, more bast-tubes to conduct organic 

 food from the leaves, more strengthening tissue (fibres, which act 

 solely as strengthening structures, and wood-tubes, which serve this 

 purpose in addition to that of conduction), and more tissue in which 

 food can be stored (e.g. the rays). 



It is obvious that this important zone of growth is most favourably 

 placed in the plant. It is well protected from external influences, and 

 well supplied with necessities of life. On its immediate inner side are 

 the wood-vessels along which the water and inorganic salts pass from 

 roots to leaves ; while touching it on its outer side is the bast through 

 which manufactured food materials are sent for distribution through- 

 out the plant. Moreover, running through the cambium are the 

 medullary rays, from which it can draw supplies of food when, as in 

 early spring, no new food is being made by photosynthesis. 



Besides the functions mentioned above, the cambium plays other 

 parts under certain conditions. It is involved in the processes of 

 pruning, budding 1 , grafting, propagation of plants by cuttings 

 (" slips "), and in the healing of wounds on the stem (Ch. XVI.). 



213. Cork and Bark. It is evident that the internal formation 

 of secondary wood and bast must exert considerable pressure on the 

 outer tissues, which are stretched and eventually ruptured. To 

 provide for this there is the formation of new tissue from a growing 

 layer developed in this region. This layer is the cork-cambium. In 

 the majority of cases it arises in the outermost layer of the cortex, im- 

 mediately below the epidermis. The young cells given off to the outer 

 side of the cork-cambium form the cork. This tissue, being imper- 

 meable to water, cuts off the epidermis from nourishment. The epi- 

 dermis dies and peels off as the first bark of the tree. The cork is 

 developed to replace the epidermis and carry on its functions. 



214. The Bark may be denned as the dead tissue lying 

 outside an active cork- cambium. We have already indicated 

 what the first bark consists of. The first cork-cambium 

 formed may persist for a large number of years, or even 

 throughout the life of the tree. In such cases, e.g. Birch, 



