164 BOTANY PART i 



peculiar white colour of Birch-bark is caused by the presence of granules of 

 betulin (birch-resin) in the cells. 



Healing of Wounds ( 83 ). 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 impregnated with protective substances and sometimes also form 

 suberised lamellae. In the case of larger wounds in the Phanerogams a cork- 

 cambium forming WOUND-CORK develops below these altered cells. Thus the leaf- 

 scars left by the fall of the leaves (p. 119) are in the first place protected by the 

 lignification and suberisation of the exposed cells, and later by the development of 

 a layer of cork that becomes continuous with that covering the stem. The open 

 ends of the vessels in the leaf-scar become occluded with wound-gum or thyloses 

 or both ; the ends of the sieve-tubes become compressed and lignified. 



When young tissue is exposed by a wound, a formation of CALLUS usually takes 

 place. All the living cells which abut on the wound grow out and divide, becoming 

 closely approximated. The surface of the new growth may at once become corky 

 and thus afford the necessary protection. In most cases a cork -cambium forms 

 in the peripheral layers of the callus and gives rise to cork. In stems of Gymno- 

 sperms 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 growing over the wounded 

 surface, an outer protective covering of cork is developed ; at the same time a new 

 cambium is formed within the callus by 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 which is brown and dead. 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 distinguished as CALLUS WOOD. It consists at first of almost iso- 

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

 forms. In the Cherry instead of normal wood-elements nests of thin- walled paren- 

 chymatous cells which undergo gummosis (p. 39) are produced on wounding 

 the cambium. 



Restitution. Secondary tissues often take part in the process of 

 restitution, i.e. the replacement of parts that have been lost. 



In the more highly organised plants the direct replacement of lost parts is 

 extremely rare. It occurs most readily in embryonic organs, such as growing 

 points, when portions have been lost, and is most often found in seedlings. Thus in 

 seedling plants of Cyclamen even a severed leaf-blade has been found to be replaced. 

 As a rule, however, when regeneration processes are requisite in higher plants, and 

 the necessary preformed organs are not present in a resting or latent condition, the 

 older tissues return to the embryonic condition and give rise to new growing points 

 of shoots. Since this provision for the indirect replacement of lost parts exists in 

 plants, the fact that direct regeneration is far more frequent among animals than 

 plants is-readily comprehensible. 



