ORGANIZATION OP THE STEM. 55 



shrubs, the bodies of which are of a firm and vigorous texture, 

 it is a part of little importance, excepting in the young and 

 tender state of the plant; but in reeds, grasses, and other 

 plants with hollow stems, it is of great use and is exceedingly 

 strong, being chiefly composed of silica, or flint. The epidermis 

 is not represented in this section. 



2. The epiphlceum (exi upon, $71010$ bark), or corky envelope, 

 shown in the section at h h. This is the outer covering of the 

 bark, and consists of cubical or flattened tubular cells, without 

 chlorophyl, placed close together and elongated in a horizontal 

 direction. It is this part of the bark which gives to the trunks 

 of trees their peculiar color and rugged appearance ; generally 

 some shade of ash-color or brown. 



In Quercus suber, the cork oak, the epiphkeum consists of 

 numerous strata of cells, forming the substance called cork ; 

 hence the name corky envelope, which is given to it. So also 

 the branches and branchlets of Liquidambar styraciflua, the 

 sweet gum-tree, and of Ulmus racemosa, one of the elms of the 

 northern United States, are winged with corky ridges, the 

 result of an unusual development of the epiphlceuni of the bark. 

 In the currant and birch, the epiphloeum is composed of only 

 a few layers of cells, and may be seen peeling off in thin cir- 

 cular pieces from the trunks of these trees. When the 

 epiphloaum is very thick, it is simply fissured or rent, in which 

 state it remains attached to the outside of the stem, forming an 

 excellent protective envelope to the inner and vitally active 

 layers of bark. 



3. The mesophlceum or cellular envelope, represented at g. g. 

 This lies immediately on the outside of the liber. Its cells 

 contain chlorophyl, and are developed vertically. It is, there- 

 fore, that part of the bark which is colored green, and which 



