1 64 THE PHYSIOLOGY OF PLANTS 



been paid by gardeners, is the time of the day at which the 

 operation should be performed. It has been usual to prefer 

 the springtime for this operation, because it seemed desirable 

 to avoid the great heat of the summer. It was supposed that 

 the latter would dry up the wound too rapidly, and therefore 

 dull days were selected for this operation, or the wounds 

 artificially shaded or protected. But these assumptions are 

 wrong and have led to frequent failure. We have seen that 

 the exposed area protects itself by the thickening of the outer 

 cell-walls or by the collapse of these cells. Now both these 

 processes are accelerated by light and heat. Long-continued 

 moisture, on the other hand, will facilitate the attacks of parasitic 

 Fungi. It is therefore best to peel trees in hot summer weather 

 immediately before the second growth in August, and to do so 

 in midday heat. 



§ 33. Why do we slit the bark ? 



By slitting the bark we mean making longitudinal incisions, 

 in which the knife penetrates into the region of new wood, 

 but does not remove auy of the tissues. These incisions relieve 

 temporarily the pressure of the bark upon the wood in the 

 regions which are cut, and prevent the trees from becoming 

 hide-bound. The wounds thus inflicted heal very rapidly by 

 a closing in of the two margins of the cut, and in accomplish- 

 ing this a certain amount of the substances stored in the wood 

 is used up. 



This operation may therefore be undertaken either when the 

 cortical pressure in stem or branch becomes so great as to 

 prevent the formation of the necessary amount of new wood, 

 or when more plastic material or sap is stored up in the stem 

 than can be used up. 



The pressure of the bark can also become harmful when 

 the natural shedding of the bark of old trees is retarded. The 

 development of the bark in the cases of those trees which 

 present in their old age a rough bark is somewhat as follows : 

 — First of all, the stem in its young and succulent condition 

 is surrounded by a simple epidermis, consisting of parenchy- 



