214 



PART IT. THE INTIMATE STRUCTURE OF PLANTS. 



[ 36. 



cork precisely similar in structure to the ordinary rind. In plants 

 in which the wood is well developed, cork is not immediately 

 formed particularly when the cambium is wounded or laid bare 

 but all the living cells which border on the wound become meris- 

 matic and give rise to a homogeneous parenchymatous tissue 

 known as the Callus. If the wound is small, the call us- cells pro- 

 ceeding from the different sides soon come into contact and close 

 up into a single mass of tissue, which then gives rise to cork on 

 its outer surface, and, joining the old cambium at the margins, 



forms a new layer of cam- 

 bium which fills up the 

 cavity. If the wound is a 

 large one, cork and new 

 cambium are formed in the 

 callus at the margins of the 

 wound, and it is not wholly 

 closed till after repeated 

 rupture of the approaching 

 cushions of callus. The wood 

 exposed by the wound, which 

 usually assumes a dark colour 

 under the influence of the 

 air, does not grow with that 

 formed from the new cam- 

 bium of the callus ; hence in- 

 scriptions, for instance, which 

 are cut in the cortex so as to 

 reach the wood, though sub- 

 sequently covered by a num- 

 ber of annual layers of wood 

 corresponding to the number 

 of years, may easily be found. A similar explanation accounts for 

 the fact that the surfaces of the stumps of cut-off branches become 

 overgrown ; the callus first appears as a ring from the cambium 

 exposed in the tranverse section, and afterwards closes like a cap 

 over the old wood. Foreign bodies nails, stones, and stems of 

 other plants may thus become enclosed in the wood of a tree and 

 be overgrown by it ; the cortex, being forced against the foreign 

 object by the pressure of the growing wood, splits, and the callus 

 formed in the rent grows round the object, enclosing it and pro- 

 ducing' new cambium. 



FIG. 158. Diagrammatic longitudinal section 

 of a woody stem : A a short time after the 

 amputation of a lateral branch s; B when the 

 wound is completely closed ; r cortex ; c cam- 

 bium ; 7i wood ; c' position of the cambium- 

 layer at the time of amputation ; 7i/ wood formed 

 since the amputation ; w the cushion of callus 

 formed over -the surface of the wound. 



