THE STEM. 49 
53. In the newly formed layers of bark, the sap, which 
hhas been modified in the leaves by the action of the air, de- 
‘scends to nourish and promote the growth of the plant. 
‘The outer bark also serves the purpose of protecting the - 
new layers of wood and bark from injury. The old and 
hardened layers form an excellent protection from external 
violence. 
154. The bark, being the part in which the sap descends 
to supply the plant, is essential for its increase. If part of 
the bark be removed from a tree all round, so as to leave 
the wood bare, the part beneath will not grow, the medium 
by which the nutritious fluids ‘were conveyed to it having 
been removed, and the tree will ultimately perish. A graft 
will not take if its bark be not in contact with that of the 
tree in which it is inserted ; and a branch will not take root 
when surrounded with earth, if the part be deprived of its 
bark, 
155. The bark is easily renewed when it has been destroy- 
, if the injury be not too severe, or too large a quantity re- 
oved ; while the renovation is going on, it requires to be 
otected from the access of the air. The renewal of the 
bark takes place by means of the cambium, which exudes 
from the wood and edges of the wound, and gradually re- 
pairs the injury. ee 
_ 156. The bark not permitting much distension, its fibres 
are generally ruptured or separated, so as to present a sort 
of net-work appearance, as in the Ash or Willow. In the 
Daphne lagetto, or lace bark tree, the fibres assume the ap- 
pearance of lace. 
157. The medulla, or central part of the tree, and the 
bark, are connected by the rays of cellular tissue (medullary 
ing lines, and serve for the horizontal diffusion of he e- 
pokes juices. a 
: 4. The Wood. ae 
Tip rod lies oneal, under t the eat, and 
ys), which stretch horizontally from the medulla in diverg- : ree 
