162 BOTANY part i 



tion and suberisation of the walls of the exposed cells, and later by 

 the formation of a layer of cork continuous with the periderm cover- 

 ing the stem. The ends of the tracheal elements in the scar become 

 occluded by wound-gum or tylosis or by both ; the ends of the sieve- 

 tubes become compressed and lignified. The leaflets of compound 

 leaves usually become separated from the rhachis by absciss layers. 

 Herbaceous plants exhibit as a rule no definite leaf-fall. Al^sciss layers 

 formed in a special way are also found at the base of the leaf-stalks 

 in ferns, which shed their leaves. The cells covering the leaf-scar then 

 form a protective layer by the deposition of lamellae which become 

 suberised, and by becoming filled with certain substances (^^-). 



Healing" of Wounds (^^''^). — 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 sub- 

 stances and sometimes also protected by suberised lamellae (^^■*). The 

 protection of wounded surfaces takes place in this way in the Cryptogams, 

 and when the Avound 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 {^•'^). 

 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 (^^^). Laticiferous elements are first 

 closed by the coagulation of the latex at the point of wounding and 

 below this a cell wall is formed (^^''). 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 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 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 siu-face, an outer protective 

 covering of cork is developed ; at the same time a new cambium is 

 formed within the callus, by the difterentiation of an inner layer of 

 cells, continuous with the cambium of the stem. When the margins of 

 the overgrowing 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 



