136 THE PHYSIOLOGY OF PLANTS 



form of cut and the proper time for performing it. The form 

 of cut to be chosen must be such that the atmospheric moisture 

 and the water which exudes from the branch are prevented 

 from collecting on the cut. If we cut oiF vertical branches 

 horizontally, the wound will be a horizontal one. But every 

 cut branch forms, owing to the greater contraction of the pith, 

 a cup-shaped depression at its cut end, and this depression 

 retaining the rain-water, dew, or the sap due to the bleeding 

 of the branch for a considerable time, will enable the germi- 

 nation of fungal spores to take place, with the result that the 

 fangal hyphce will destroy the tissues. A wound with an oblique 

 position will therefore be less harmful, and all transverse cuts 

 should be diagonal. 



The precautions we have to take to avoid the collecting of 

 moisture will also indicate that we should not choose as a 

 season for pruning the time when plants " bleed," that is, force 

 out much water from their tissues. Although the wounds will 

 be found to be provided with means of preventing the giving off 

 of water, we often see old wounds giving off a considerable amount 

 of water. The means of protection are of two kinds. In the 

 parenchymatous tissue a layer of separation is formed extending 

 from the cork right across the cortex. The vessels become closed 

 up by plugs of a peculiarly resistant gum-like mass, or by the 

 bulging out into the vessels of the surrounding parenchymatous 

 cells. These vascular protrusions are forced through the pits 

 until they meet each other, and so close the vessel (thylloses). 

 All these protective formations, however, arise more rapidly in 

 the younger tissues and in those which contain a large supply 

 of plastic substance. If we cut off a branch in a green 

 herbaceous condition, we notice that the shoot dies down as 

 far as the next bud, and here a layer of cork will be formed 

 separating the dead from the living tissues. In stronger 

 shoots, in which the wood cylinder is better developed, 

 a swelling will often be noticed immediately below the dead 

 tissues. Such a swelling occurs in many kinds of fruits, and 

 is no disturbing sign ; it is formed by an enormous increase of 

 parenchymatous tissue in proximity to the hard bast fibres, 

 which often die away for a considerable distance in the living 

 tissue and become enveloped by a layer of cork. 



