PHYSIOLOGY. 967) 
Hence it is that the under part ef the petiolus re- 
mains and forms the bark. : 
Gr26il, 
{f the vascular bundles of a tree or shrub remain 
in a straicht direction, the stem ascends without 
sending out any branches. ‘The new shoots in the 
hazel, (Corylus Avellana), Berberis vulgaris and 
all which the trunk of trees produce when lopped, 
are a proof of this. As soon, however, as the air- 
vessels become convoluted, and form a knot, branches 
are formed. By assistance of art such straight shoots 
' may be brought to branch, by making a transverse 
incision through their bark. ‘The separated air-ves- 
sels heal the lips of the wound, are several times 
convoluted, and growing larger are obliged to form 
more gems from which branches arise, 
§ 262. 
The growth of ligneous plants admits of five va- 
rleties : 
1. Trees and shrubs, (Arbores et frutices), have 
their stems beset with leaves. On the base of each 
petiolus a bud or gem is formed, which again 
becomes a leafy branch, provided with gems of 
the same kind, and undergoing the same changes. 
If the main shoot grows at first in a straight line to 
a certain height without the buds on its sides being 
able, on account of the too hasty circulation of the 
sap, to form themselves into branches, or these, 
should they really be formed, not able to grow any 
more, such a plant then becomes a tree, which has a 
straight 
