86o 



We consider the curly or gnarly wood only as an extreme case of per- 

 fectly normal processes, in the variation of the wood fibres when obstacles 

 occur which prevent their longitudinal arrangement in the part of the plant. 

 Such obstacles can differ greatly. Each normal branch insertion becomes 

 the cause of a change in the course of the wood fibres surrounding it. The 

 new formation of wood bodies within the bark, described under bark tubers, 

 represents a further cause. Finally, however, we find the most varied 

 phenomena of arrestment in the formation of an annual ring, produced by 

 differences in tension in the growing axis. Such differences in tension are 

 constantly present and are often strengthened by external influences. Frost 

 action, for example, which causes the formation of parenchyma bands, is 



Fig-. 204. Gnarlly wood sliucture of the ovGrgrowth cap of 



branch. 



of especial significance. Another external cause is the contact' of one 

 branch with another. Besides mechanical pressure, conditions of light are 

 also of influence ; they cause variations in the nutrition of the different sides 

 of the cambial ring. Internal processes of growth, as, for example, the 

 rapid outpushing of a suddenly broadened medullary ray, are also of impor- 

 tance. These can tiistend the bark into knobs, causing a repression in the 

 growth of the adjoining wood layers and the like. All such disturbances 

 must change the pressure conditions which the bark girdle in its entirety 

 exercises on the cambium and will, therefore, influence the development of 

 the wood formed from it. We find in the spiral twisting of the wood body 

 in every trunk, hew greatly the course of the fibres is influenced by the 



