48 THE STEM. 
2. Herbaceous or Cellular Integument. 
149. This is the layer of cellular tissue, which les mm 
diately under the epidermis, and gives to the leaves 
young stems their green colour. It forms the substance 
the leaf, and in it the changes effected on the sap. by 
atmosphere take place ; hence it frequently contains the 
per juices. It is easily repaired on the young stems of ¥ 
_ vegetables, but never on those of annual plants or on | 
_ The green colour does not arise from the sides of the 
but from the presence of small granules of globuline. 
3. Liber, Cortex, or Bark. 
150. This is found immediately under the herbaceous 
tegument, and consists of a network of woody tissue, 
spaces between the vessels being filled up by cellular 
Spiral vessels are not found in the bark, except in — 
thes. Air vessels, and the form of int 
termed vasa propria, abound in the bark. 
151. The bark consists of concentric layers, odeli 
ymposed of a layer of woody tissue and one of cellular 
= Pioee Welly Gos eer of tach in young shoots one ; 
and the layer of cellular tissue is external, or 
other. A new layer of each is formed in each su 
year within the old one; and thus, in trees, the bark 
up of as many woody layers as the tree is years ¢ 
older layers being pushed outwards and destroyed 
growth of the new layers, and becoming a lifeless 
—— the bark may be called 
be thrown off like the patches of bark, after a year 
_ 152. The proper juices and secretions of the 
chiefly in the bark, in the innermost layers al 
principally from this part that we extract 
