178 THE PHYSIOLOGY OF PLANTS 



wlien the tip of a shoot is used as a cutting, its callus and 

 adventitious roots are not formed from reserve food matter 

 which has been stored in its tissues at some previous period, 

 but that the materials necessary for these growths have to be 

 formed by the cutting after separation from the parent plant. 

 As soon as a portion of a plant is dependent for its food upon 

 its leaves, we kngw that it needs light. Herbaceous cuttings 

 therefore need light, and comparatively much light, while 

 woody cuttings can do with very little light, at the outset 

 at least. 



The herbaceous cutting is taken before its axis is much 

 lignified. The cut surface exposes tissues which have as yet 

 no thickened cell-walls ; the cells are rich in protoplasm and 

 cell sap, are more prone to changes and decomposition, and 

 require therefore an increased stimulus to continue the vegeta- 

 tive processes in spite of the wound which has been inflicted. 

 This stimulus is provided by the increase of temperature. 

 Herbaceous cuttings require, therefore, more heat than cuttings 

 of the same species taken from older portions of the plant. 

 In some cases indeed the temperature requisite for herbaceous 

 cuttings is harmful for the woody cuttings, because it calls forth 

 certain changes (possibly of a fermentative nature) the products 

 of which cannot be used up at the time, and therefore cause 

 decay. 



We must remember, on the other hand, that the wound itself 

 cannot be healed at once, and that the soft cuttings lose con- 

 siderable amounts of water from their leaf surface by transpira- 

 tion, and this at the time they are without roots, which could 

 supply the requisite amount of water. We must, therefore, 

 reduce in the first instance the transpiration without taking 

 away the leaves. This can be done either by shading the 

 cuttings or by keeping the air saturated with moisture. Every 

 decrease of the amount of light diminishes also the amount of 

 transpiration. In a damp or saturated atmosphere, too, the 

 transpiration of the leaves is reduced. 



Herbaceous cuttings require, therefore, at the outset a moist 

 atmosphere. We purposely say at the outset, because it is a 

 frequent source of error to continue this for too long a time. 

 Absence of light and a large amount of atmospheric moisture 



