THE STEM OF VASCULAR PLANTS. 185 



in diameter, we connect, in reality, two distinct pheno- 

 mena — viz. the increase or extension of layers already 

 existing, which he calls their increase in width (largeur), 

 and the addition of new layers, which he calls their 

 increase in thickness, (epaisseur.) The increase in 

 diameter may result sometimes from the union of these 

 two phenomena, and sometimes from the existence of 

 only one. 



According to this observer, the extension of layers 

 already existing takes place, both in the cortical system 

 and in the central one, by an analogous process ; and 

 that, as regards the bark, it can easily be followed in 

 autumn in the root of Echium vulgare; and as to the 

 woody body, in spring, in the young shoots of Clematis 

 Vitalha. In both, on cutting them at different heights, 

 in order to have the comparison of different ages, we 

 see that the horizontal section of a layer presents a 

 certain number of fibrous bundles, separated by vertical 

 plates of cellular tissue — the medullary rays ; that these 

 rays are, at certain periods, divided into two plates by 

 a row of fibres which are developed in the middle, and, 

 gradually separating them, begin by forming kinds of 

 festoons, and afterwards two distinct medullary rays ; 

 and that, lastly, these bundles of longitudinal fibres 

 develope in their middle new medullary rays. This 

 formation explains how there are many more medullary 

 rays in the layers on the border of the woody body than 

 in those of the centre, — as can be easily seen in the sec- 

 tion of the branches of Quercus toza, (pi. 5, fig. 3.) 

 Analogous phenomena take place in the bark and woody 

 body, and cause us to understand the increase which the 

 already existing layers take. The formation of the 

 medullary rays, the primitive and secondary ones of the 

 woody body, is in accordance with the angles of the 

 medullary canal ; and that of the cortical rays is with the 



