112 ORGANIC STRUCTURE. 



whatever way the wedge is introduced, it is the 

 cellular matter that yields in the cleavage. By no 

 power can the trunk be cloven transversely ; the 

 fibrous structure opposes all efforts to separate the 

 wood across its position. 



The longitudinal strength, common to all woody 

 stems, appears, however, to be in some plants acquired 

 by age. The grape vine is an instance among trees 

 of a peculiarity in the structure of the stem when 

 very young, namely, that the longitudinal arrange- 

 ment of the fibres is interrupted at every joint, part- 

 ing easily at the nodes, leaving a clean surface on 

 both divisions. This shows that the first layer of 

 wood is somewhat articulated, and has less longitu- 

 dinal connection and tenacity than those which are 

 afterwards imposed. 



The woody fibres which give strength and tough- 

 ness to the layers are united irregularly with each 

 other ; in young shoots which have just commenced 

 growth, some of the fibres, especially in the medullary 

 sheath, are seen disposed in single or double coils, 

 which become almost straight when lengthened out 

 in the growth, and the whole then appears (when the 

 layer is split through the middle downwards) like the 

 meshes of a net, when intensely stretched in one 

 direction. No spiral vessels are found in perfect 

 timber; because the single or double fibres which 

 formed them are then disposed at full length. 



