154 THE PHYSIOLOGY OF PLANTS 



different regions of the stem or branch are nourished in 

 different degrees. If above the wound there are plenty of 

 leafy shoots which are actively producing plastic material, the 

 descending food material will enable a rapid covering in of the 

 wound to take place. 



The shape of the wound inflicted is also of some importance. 

 If we examine, for instance, the place where a piece of wood 

 has been cut out of a stem, we notice that the healing 

 tissue is more profusely produced at the upper end of the 

 injury. This is due to the fact that the plastic substances 

 descend from the leafy portion of the tree, and the upper edge 

 of the wound arrests this downward current and causes there 

 an accumulation of plastic material. If the wound is pro- 

 portionally very broad, a large amount of this material will 

 accumulate, and the healing will be more rapid than if the 

 greatest length of the wound were parallel to the long axis of 

 the stem. 



With regard to the rapidity of healing, it is interesting to 

 notice the difference between a transverse and longitudinal cut. 

 If a branch is cut across transversely, there remains of course 

 no leafy tissue above it, and the cambium ring must draw all 

 the material it needs for callus formation from the immediate 

 vicinity. But plastic material does not travel upwards to any 

 great distance, and hence it will be observed that comparatively 

 small transverse wounds will not heal, or only do so very slowly, 

 if they are far removed from a lateral bud. If you desire to 

 cut a branch transversely, it is desirable to do so immediately 

 above a bud, because in this region there is a larger store of 

 food material, and also when the bud grows out into a shoot, 

 it will send down more food material which will become avail- 

 able for the healing tissues. 



From these considerations we may deduce the general rules, 

 that wounds are covered up more readily, the less numerous 

 they are on any axis, the smaller their area, the more they 

 depart from the form of a transverse cut, and the greater the 

 amount of active leaves which remain above them. 



Any artificial aid in the process of healing must therefore 

 be based on these considerations. Hence we generally prune 

 a branch by a long oblique cut close above a bud. It is 



