146 GENERAL BIOLOGY 



number of the bundles and by their consolidation in a ring 

 of wood underlying the cambium. This will be understood 

 from a study of the stem of any woody plant, such as the 

 box elder (fig. 92). The cambium is more abundant, and 



Fig. 92. Segment of a four year old woody stem, 

 with bark in part removed (b); c, cambium; p, 

 pith; /, 2, s, 4, wood of the four years growth; 

 vertical surface shows wood fibres overlaid by 

 transverse fibres of the medullary rays. 



clearly delimits bark and wood. In the bark there is a 

 copious development of bast fibres, that protect underlying 

 protoplasmic parts (fig. ()ic). The vessels of the wood are 

 pitted vessels, and not spiral; and, most striking of all, 

 the bundles are very numerous and very closely crowded 

 together. Obviously such weak and isolated vessels as 

 those of the chickweed, while capable of conducting water, 

 are of little use for support. 



The pitted vessels constitute the frame work around which 

 is built the woody skeleton of the box elder tree. Wood, as 

 we ordinarily know it, is composed of these vessels and of 

 wood fibres, and wood fibres are made out of the paren- 

 chyma cells which we" have hitherto seen forming the pack- 

 ing around and between the bundles. The cells lying be- 

 tween the vessels become elongated, lignified in their walls 



