THE US:^ OF SHOOTS FOR PROPAGATING 173 



provided with leaves, while in the case of the former it is not 

 necessary. Woody cuttings always form callus over their 

 cut end ; in herbaceous cuttings it need not be formed. The 

 formation of roots in a cutting is not dependent upon callus 

 formation. 



We may here repeat again that callus is a thin- walled colour- 

 less tissue, consisting of meristematic cells arranged in close 

 rows, of which the end ones are still in process of growth and 

 which have not as yet become differentiated into cork or 

 wood. 



The first sign of life in a cutting manifests itself by an 

 alteration of the tissues near the cut surface, the cut generally 

 running obliquely across the shoot, and being close below a 

 bud. If we cut off a shoot, we thereby expose all the tissues of 

 which it is formed, and we then bring the latter in contact with a 

 damp medium (water, sand, earth, sawdust, fibre, &c.). Some of 

 the tissues which have been exposed are not able to form the 

 healing layers of callus ; this is always the case with the old 

 wood, often with the pith and the outermost layers of the 

 cortex. The layers which are capable of further division, and 

 are therefore charged with the production of the protective 

 callus, ajre the cambium, the very young wood cells, and the 

 innermost layers of the cortex. The larger, therefore, the area 

 of exposed wood as compared with the other tissues, the more 

 difficult will be the healing process. 



It is therefore essential for the success of propagation by 

 means of cuttings to bring about a sufficient and natural clos- 

 ing up of the cut end of the shoot. 



This closing takes place by two processes. In the older 

 soft tissues (pith and old cortex) there will be formed above 

 the wounded cells transverse layers of cork cells which protect 

 the cutting against excessive moisture. The woody elements 

 adjoining the damaged wood cells and vessels may become 

 plugged up with a very resistant brown mass (gum) or with 

 thylloses, which have the same effect in closing the apertures 

 of these cells and vessels. The second process is the covering 

 in of the cut by the formation of callus. 



Both processes take place (with very few exceptions) more 

 completely when the cut surface is richly supplied with air. 



