THE 3TEM OF VASCULAR PLANTS. 171 



§ 3.— Of the Cellular Envelope. 



On the outside of the cortical layers is found a zone 

 of cellular tissue, which bears the name of the Cel- 

 lular Envelope (enveloppe cellulaire). It is a kind 

 of external pith ; if it be examined in its young state, 

 it presents, like the pith, a round regular cellular tissue, 

 only differing from it in its position and colour. The 

 position appears very different ; but if a little more 

 attention be paid to it, it is in reality very similar ; for, 

 in starting from the line which separates the woody from 

 the cortical body, we see succeeding each other in regular 

 order, the alburnum, the wood, and the pith, on one 

 side ; the liber, the cortical layers, and the cellular enve- 

 lope, on the other. The colour is in accordance with its 

 position : the pith, which is screened from the light, is 

 white ; the cellular envelope, which is evidently sub- 

 mitted to its action, is green. In several fleshy plants, 

 or those v.ith a loose tissue, as the Mistletoe, these two 

 organs present a greater analogy ; and we shall see, by- 

 and-by, communications from one to the other. Du- 

 trochet also confirms this analogy between the pith and 

 cellular envelope, by showing that the pith can, in cer- 

 tain cases, like the cellular envelope, form, when it is 

 laid open, a true epidermis ; he designates them, in 

 consequence, by the names of the Central Pith (medulle 

 centrale) and External Pith {medulle exterieure). 



The cellular envelope of the shoots of the same year, 

 is green, regular, and entire ; in the second year, it 

 begins to be distended by the growth of the stem. It 

 resists this distension more, according as the growth of 

 the trunk is less rapid, or as the cellular envelope itself 

 is more flexible, and, consequently, more extensible. As 



