124 DISEASE IN PLANTS. 



divide into a juicy mass of thin-walled cells 

 which is of a cushion-like nature and is termed 

 a Callus. This callus is at first a homo- 

 geneous tissue of cells which are all alike 

 capable of growing and dividing, but in course 

 of time it undergoes changes in different parts 

 which result in the formation of tracheids, vessels, 

 fibres and other tissue-elements, and even organs, 

 just as the embryonic tissues of the growing 

 points, cambium, etc., of the healthy plant give 

 origin to new growths. Such wound-wood, how- 

 ever, is apt to differ considerably in the arrange- 

 ment, constitution and hardness of its parts as 

 compared with normal wood, and its peculiar 

 density and cross-graining are often conspicuous. 

 If instead of a simple tissue, the cut or other 

 wound lays bare a complex mass such as wood, 

 the resultant changes are essentially the same to 

 start with. The living cells bordering the wound 

 form cork, and then those deeper down grow out 

 and form a callus. The exposure of the wood 

 however, entails alterations in its non-living 

 elements also. The lignified walls of tracheids, 

 fibres, etc., turn brown to a considerable depth, 

 and this browning seems to be ^like all such dis- 

 colorations in wounds due to oxidation changes 

 in the tannins and other bodies present : the 

 process is probably similar to what occurs in 

 humification and in the conversion of sap-wood 

 into heart- wood in trees. Such wood is not 

 merely dead, but it is also incapable of con- 

 veying water in tlie lumina of its elements, which 



