124 THE ELEMENTS OF VEGETABLE HISTOLOGY 



cal row (Plate 46, No 1). The older sieve plates are 

 apt to show extremely thick perforated end walls 

 and may contain triangular patches of solidified cell 

 contents. The walls of sieve tubes are composed of 

 cellulose showing no traces of lignification. In gen- 

 eral, the sieve tubes conduct materials away from 

 the leaves, the substances transported including nitro- 

 genized nutrients and carbohydrates in solution. 



Medullary Rays. — The ducts and sieve tubes pro- 

 vide means for the transportation of materials up 

 and down the stem, and in younger plants communi- 

 cate directly or indirectly with all the cells. As the 

 stem increases in thickness it becomes necessary to 

 provide conducting tissues to transport nutrients to 

 the cells located at some distance from the narrow 

 zone in which the ducts and sieve tubes are placed. 

 This work of lateral conduction is performed by 

 medullary ray cells which extend from the cambium 

 into the bark on one side and into the woody tissues 

 on the other. The first medullary rays are strips 

 of the original parenchyma between the vascular 

 bundles formed in the plerom region, and they extend 

 from the center of the stem to the inner layers of bark 

 without interruption except in the cambium zone. 

 These primary rays are soon supplemented by second- 

 ary rays resulting from the formation of new vas- 

 cular bundles in the widest primary rays. The walls 

 of medullary ray cells are usually thin and are com- 

 posed of cellulose. In many instances the cellulose 

 walls show numerous pores, thus increasing the effici- 

 ency of the ray cells as distributing elements. In 

 the non-porous type of ray cells materials are trans- 

 ported from one cell to another by dialysis through 



