144 BOTANY 



addition, they, as well as the ends of the sieve-tubes, become com- 

 pressed and finally cut off by the developing cork. 



Wounds. — In the simplest cases the exposed tissues of wounded 

 surfaces become dry through loss of moisture, and dying in consequence, 

 form over the deeper-lying tissues a protective covering of dry, brown 

 •cells. This method of protecting wounded surfaces, although very 

 general in Cryptogams, rarely obtains in Phanerogams, but instead 

 the wounds become closed by the formation of cork. Cork formed 

 over wounded surfaces is called wound cork. It is derived from 

 a, cork cambium that develops in the tissue under the wounds, and 

 with its development the process of healing, in succulent and paren- 

 chymatous portions of plants, is completed. In woody plants a so-called 

 callus is formed by the active growth of the living cells bordering on 

 the wound. These abnormal swollen growths close together over the 

 wound, and by the suberisation of their cell walls provide a sufficient 

 protection. Generally, however, a cork-forming phellogen arises in the 

 periphery of the callus. In stems of G-ymnosperms and Dicotyledons, 

 wounds which extend into the wood become surrounded and finally 

 overcapped by an outgrowth of tissue arising from the exposed 

 cambium. While the callus tissue is still in process of gradually 

 overgrowing the wounded surface, an outer protective covering of cork 

 is developed ; at the same time a new cambium is formed within the 

 callus, through the differentiation of an inner layer of cells, continuous 

 with the cambium of the' stem. When the margins of the over- 

 growing callus tissue ultimately meet and close together over the 

 wound, the edges of its cambium unite and form a complete cambial 

 layer, continuing the cambium of the stem over the surface of the 

 wound. The wood formed by this new cambium never coalesces 

 with the old wood. Accordingly, marks cut deep enough to pene- 

 trate the wood are merely covered over by the new wood, and may 

 afterwards be found within the stem. In like manner, the ends of 

 severed branches may in time become so completely overgrown as to 

 be concealed from view. As the wood produced over wounds differs 

 in structure from normal wood, it has been distinguished as CALLUS 

 WOOD. It consists at first of almost isodiametrical cells, which are, 

 however, eventually followed by more elongated cell forms. 



The Formation of Burrs. — The curled or extraordinarily knotted 

 appearance of wood, such as the bird's-eye or curled maple, which adds 

 so much to its technical value, is due to the unusually sinuous course 

 taken by the elements of the wood. This variation from their usual 

 direction is caused by the development of numerous adventitious 

 buds, which turn the vascular bundles out of their accustomed course'; 

 the direction of the wood elements is moreover often affected by the 

 medullary rays, which sometimes become so abnormally swollen that 

 they appear almost circular in tangential sections. 



