MORPHOLOGY 151 



to parts of the vascular bundles. In the simplest cases among 

 land plants the wounded cells die and become brown and dry, 

 while the walls of the underlying uninjured cells become lignified. 

 The protection of wounded surfaces takes place in this way in the 

 Cryptogams and when the wound is very small in Phanerogams. 

 In the case of larger wounds in the latter a cork-cambium, forming 

 wound-cork, develops below the lignified cells. This cork is lignified 

 as well as suberised ( 14 ). Suberisation of the walls of the cells 

 immediately underlying the wound may precede the formation of 

 cork. This occurs in seed potatoes which are divided up before 

 planting and left one or two days exposed to the air. On this 

 depends their resistance to injurious influences in the soil ( 141 ). 

 Secretory receptacles in wounds become closed by thyloses. Latici- 

 ferous elements are first closed by the coagulation of the latex at 

 the point of wounding and below this a cell-wall is formed ( 142 ). 

 In woody plants a so-called CALLUS is formed by the active 

 growth of the living cells bordering on the wound. These swollen 

 growths close together over the Avound, and by the suberisa- 

 tion of their cell-walls provide a sufficient protection. Generally, 

 however, a cork-forming phellogen arises in the periphery of the 

 callus. In stems of Gymnosperms 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 a 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 penetrate 

 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. The growing points of adventitious shoots 

 often arise in such masses of callus. As the wood produced over 

 wounds differs in structure from normal wood, it has been distin- 

 guished as CALLUS WOOD. It consists at first of almost isodiametrical 

 cells, which are, however, eventually followed by more elongated cell 

 forms. At the base of cut-off portions of plants which are used as 

 cuttings a callus formation which according to the kind of plant is 

 more or less active, takes place. In extreme cases the callus forms pro- 

 jecting masses within which the rudiments of adventitious roots arise. 

 Regeneration. Lost parts of the body of Fungi and Algae are 



