76 



MORPHOLOGY OF STEMS. 



tal system of the wood, or the woof, into which the vertical woody 

 fibre, &c., or warp, is interwoven. The inspection of a piece of oak 

 or maple wood at once shows the pertinency of this illustration. 



3rd. The Bark or rind. This at first consisted of simple 

 parenchyma, like that of the pith, except for the green color 

 developed in it,-the same as that which gives verdancy to foliage. 

 This green matter is formed in the cells of all such parts when 

 exposed to light, consists of green grains of somewhat complex 

 chemical composition, has important functions to perform in 

 assimilation (i. e. in the conversion of the plant's crude food into 

 vegetable matter), and is named CHLOROPHYLL, i. e. leaf-green. 

 The completed bark, when all its parts are apparent, as espe- 

 cially in most trees and shrubs, is composed of three strata, of 

 which the green bark, the most conspicuous in the young shoot, 

 is the middle laj-er, therefore named the MESOPHLCEUM. This is 

 soon covered, and the green color obscured, by a superficial 

 stratum of cells, generally of some shade of ash-color or brown, 

 occasional!}' of brighter tints, which gives to the twigs of trees 

 and shrubs the hue characteristic of each species, the CORKY 

 ENVELOPE or layer, or EPIPHLCEUM. The latter name denotes its 

 external position ; the former, that it is the layer which, when 

 much developed, forms the cork of Cork-Oak and those corky 

 expansions which are so conspicuous on the twigs of the Sweet 

 Gum (Liquidambar) , and on some of our Elms (Ulmus alata 

 and racemosa) . It also forms the paper-like exfoliating layers 

 of Birch-bark. It is composed of laterally flattened parenchy- 

 matous cells, much like those of the EPIDERMIS (Fig. 133, z), 

 which directly overlies it, and forms the skin or surface of the 



FIG. 134. Vertical section through the wood of a branch of the Maple, a year old, 

 BO as to show one of the medullary rays, passing transversely from the pith (p) to the 

 bark (6): magnified. But a section can seldom be made so as to show one unbroken 

 plate stretching across the wood, as In this instance. 



FIG. 135. A vertical section across the ends of the medullary rays: magnified. 



