15° 



THE PHYSIOLOGY OF PLANTS 



bast, while the more internal ones which cover the wood form 

 short lignified wood cells. Thus the meristematic callus forms 

 gradually the closing layers of wood which cover in the cut 

 surface, and which may be looked upon as a continuation of the 

 tissues which formed the branch. The chief feature which we 

 have to notice is that the cambium of the branch is continuous 

 with the cambium which forms the closing layers. Upon the 

 activity of this cambium depends the annual advance and 

 increase in thickness of the layers of wood which ultimately 

 cover the cut like a cap (Figs. 23, 24, 2$). 



The function of the callus and closing layers will be readily 

 understood from the adjoining figures. Fig. 23 shows the 

 formation of callus on the surface of a woody tissue which has 

 been laid bare ; this will take place under favourable conditions 



in all stems in which 

 the bark and bast 

 has been peeled off 

 during the period of 

 special activity of the 

 cambium. In this 

 case the youngest 

 layers of cells on the 

 surface of the wood 

 will enlarge to form 

 rows of delicate cells 

 (c) which are united 

 into a continuous layer. This tissue is formed not only 

 by the cells (m) which form the continuation of the medullary 

 rays, but also by those (/) which adjoin the wood fibres. In 

 comparison with the fully developed elements of the wood (h) 

 the cells of the newly-formed tissue are exceedingly thin walled 

 and full of protoplasm ; but they chiefly differ from the former 

 by constantly growing in length and cutting off" cells at 

 their apex. Later on this active growth in length ceases, 

 and the last-formed cells and those immediately below them 

 become divided by closely set parallel walls, which become 

 corky, and thus form a layer of corky cells (k) which protect 

 the delicate tissue. This concludes the growth of this tissue 

 as callus. Then witbin the mass (c) a new meristematic 



Fig. 23. 

 The Formation of Callus on the Exposed Wood 

 Surface of Tilia europcea. 

 (Explanation in the letterpress.) 



