1 66 THE PHYSIOLOGY OF PLANTS 



of the woody cylinder can be gathered by observing the longi- 

 tudinal slits some hours after they have been made. For while 

 the wound inflicted was at the commencement only as broad as 

 the back of the blade, a few hours after the operation the slits 

 will be twice or three times as broad. The margins of the 

 wound have become separated by the contraction of the strips 

 of bark which were previously stretched. Such a small slit 

 heals very rapidly, as the callus masses formed at the margins 

 very soon meet over the cut. The cambium layers of the 

 callus masses join, and soon complete the cambium ring which 

 was destroyed by the incision. This cambium ring does not, 

 however, at first form long wood fibres in the incisions and 

 in their vicinity, but gives rise to a large amount of wood 

 parenchyma, the cells of which are short and square. For 

 this reason the growth in thickness of the stem is more 

 rapid. 



It is only when the wound has become covered in by a new 

 cork layer, and when the latter has become continuous with 

 the general cork mantle, that the pressure again becomes 

 greater on the cambium cells, and the new wood elements 

 assume an elongate form and become like the normal wood 

 cells. Under certain conditions, the slitting of the bark, 

 together with copious watering, may cause a renewed forma- 

 tion of spring wood, even after the autumn wood has begun to 

 be formed. If the wound heals very rapidly and a long dry 

 autumn sets in, then the ring marking the year's growth may 

 show two layers of spring and two layers of autumn wood, 

 and we have an appearance of two annual rings having arisen 

 in one year. 



The bark is sometimes slit in order to cause a thickening 

 of a slender stem of a wild stock if the scion which has been 

 inserted is very thick. In this case the numerous incisions 

 which are made all round the stem commence in the scion 

 close above the juncture of scion and stock, and extend some 

 way down the latter ; thus a considerable increase in thickness 

 of the stock is obtained. In the case of grafts which have 

 been made close to the ground, and in which the scion is 

 strong and healthy but cannot develop jaroperly, owing to the 

 weakly condition of the wild stock, slitting the bark may be 



