46 DESCRIPTIVE BOTANY. 



pear rather as isolated patches of cellu- 37 

 lar tissue, arranged in lamina? of one 

 or more cells in thickness, placed 

 at right angles to the concentric woody 

 layers (fy. 37.). The cells are elong- 

 ated in the direction of the rays. 

 In some climbers, where the stem is 

 twisted, the rays are curved from the 

 centre to the circumference. 



(.62.) Bark. The layers which compose the bark, 

 are formed on a reverse plan to that of the woody 

 layers, their outer portion being chiefly cellular, and 

 their inner more vascular. The last formed or inner- 

 most, is termed the " Liber," the rest bear the general 

 name of " Cortical layers." These layers are capable of 

 greater or less distension, according to the nature of the 

 tree ; and in some cases the fibres are so far separated as 

 to represent a sort of lace-work, as in the J)n/>lim' Inyetto. 

 In the lime tree, the inner layers, when separated by 

 maceration, form the common bass, or matting, used 

 by gardeners. The outer layers of the birch, beech, 

 and other trees, are thrown off in thin membranous 

 lamina?. In oaks, elms, and a multitude of others, 

 the old bark remains in a rugged cracked state. The 

 absence of trachea; is a nearly universal characteristic 

 of the bark ; but Dr. Lindley has detected them in great 

 abundance in that of the pitcher-plant (Nepenthes dis. 

 fU/ntoriu). 



(53.) Monocotyledonoua Stems. The complete want 

 of monocotyledonous trees in our climate, has debarred 

 botanists the opportunity of examining their structure 

 so particularly as they have that of Dicotyledons ; 

 and, perhaps, even yet, the exact course of the woody 

 fibres distributed through the trunk, is not accu- 

 rately understood. It was supposed until lately, that 

 the newest fibres were placed nearer the centre than 

 the old ones, throughout the whole of their length 

 (fy. 38. ) ; but M. Mohl has recently shown that this 

 cannot be the case. He observes that the fibres cross 



