3G] 



CHAPTER II. THE TISSUES. 



155 



36. Formation of Tissue in consequence of Injury. 



When the internal tissues of most parts of plants are laid bare by 

 injury, they are gradually covered by a formation of cork taking 

 place in the outermost layer of cells which remain uninjured and 

 capable of growth. This is easily seen in injured fruits, leaves, 

 and herbaceous stems, in which the wounds that have been covered 

 by a layer of cork are distinguished by a grey-brown colour. The 

 process is very easy to observe in potato-tubers, for each portion 

 of living tissue taken from one, if only prevented from drying too 

 quickly, will soon be covered 

 over the whole surface by a 

 layer of 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 cam- 

 bium is wounded or laid bare 

 but all the living cells 

 which border on the wound 

 become merismatic and give 

 rise to a homogenous j>aren- 

 chymatous tissue known as 

 tlic Callus. If the wound is 

 small, the callus-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 cambium 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 approach- 

 ing cushions of callus. The wood exposed by the wound, which 

 usually assumes a dark colour tinder the influence of the air, does 

 not grow with that formed from the new cambium of the callus : 

 hence inscriptions, for instance, which are cut in the cortex so 

 as to reach the wood, though subsequently covered by a number 

 of annual layers of wood corresponding to the number of years, 



FIG. 131. Diagrammatic longitudinal section 

 of a woody stem : A a short time after t.e 

 amputation of a lateral branch s; JB when the 

 wound is completely closed ; r cortex ; c cam- 

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

 layer at the time of amputation ; fc' wood formed 

 since the amputation ; w the cushion of callus 

 formed over the surface of the wound. 



