142 



CALLUS 



covering the leaf-scar is interrupted only where the vascular 

 bundles are broken across, and here the vessels are usually 

 plugged by tjdoses. The detachment of flowers and the pheno- 

 menon of self-pruning, by which small branches of certain trees 

 [e.g. Poplar) are regularly shed, is due to a similar development 



of a definite abscission-layer. 



When a branch is artificially re- 

 moved, the living cells, and especially 

 those of the exposed cambium, give 

 rise to a large-celled, thin-walled tissue 

 which at first forms a ring, but often 

 gradually becomes broader till the 

 whole wounded surface is completely 

 covered, the outermost cells usually 

 becoming suberised. The tissue thus 

 formed is called a callus, and may 

 subsequently produce adventitious 

 shoots {e.g. pollarded trees) or adven- 

 titious roots {e.g. cuttings). Regions 

 of injury in the parenchymatous tissues 

 {e.g. of cortex or leaf) are commonly 

 isolated from the healthy surrounding 

 tissue by the formation of cork from 

 a cambium produced by tangential 

 divisions in the uninjured cells nearest 

 the wound. In this way the attack 

 of a parasitic Fungus maj' often be 

 localised. 



The cut surfaces of scion and 

 stock, brought into contact in the 

 process of grafting, become intimately 

 associated owing to fusion of the 

 intact cells, which is usually accompanied by some cell-division. 

 Renewed activity in the tissues of a plant may likewise result 

 from wounding by various organisms, either vegetable or animal. 

 The local enlargements produced are termed galls (Fig. 70), 

 and in their formation the, probably chemically, stimulated cells 

 may either enlarge or undergo division. 



Little or no tissue differentiation exists in galls caused by 



Fig. 69. — Diagrammatic 

 longitudinal section 

 through part of a node 

 of the Sycamore (Acer 

 pseitdoplatanus), show- 

 ing the abscission layer 

 (S.). a.x., axillary bud ; 

 p., petiole; I'., vascular 

 bundles. 



