VEGETABLE PHYSIOLOGY: COMPOUND ORGANS. 
29 
occurs when young branches sprout from the old stems of 
trees. Branches are themselves capable of developing buds 
from their axis in a similar manner to the stem; and hence the 
lateral extension of plants is entirely indeterminate. Leaf-buds, 
or gemmae , consist of scales imbricated over each other, sur¬ 
rounding a minute cellular axis, or growing 'point , which ori¬ 
ginates in the pith, and is in direct communication with the 
woody and cellular tissue of the stem. They have a special 
power of extending in length, and are generally clothed with 
leaves as they advance, and thus become branches; but in some 
cases they harden as they extend, and sharp conical projections 
are formed, which are called spines ; these manifest their origin 
by frequently bearing leaves and even buds, as in the genus 
Crataegus , and they must not, therefore, be confounded with 
prickles , which have been previously noticed, and are mere 
external concretions of hardened cellular matter. Spines are 
in direct communication with the interior of the stem, and have 
a considerable quantity of woody tissue in their structure. The 
manner in which the young leaves of plants are folded up in 
these leaf-buds is called their vernation , or foliation ; and it is a 
point of considerable importance in systematic botany. 
As regards their internal structure, vascular stems may be 
separated into two principal kinds, which are termed exogenous 
and endogenous. The stems of exogenous plants are cha¬ 
racterised by a central cellular substance, called the pith; a 
series of vessels surrounding this, called the medullary sheath; 
external cellular and fibrous rings, called the bark; an inter¬ 
mediate mass, termed wood , consisting of woody fibre ; and cer¬ 
tain thin plates, radiating from the pith to the bark, through 
the wood, called medullary rays. The stems of endogenous 
plants are known by having neither bark, nor wood, nor pith, 
nor medullary rays, distinguishable, but are composed of vasi- 
form and vascular tissue, embedded in cellular and woody tissue. 
The stem of cellular plants is composed by the union of the 
bases of the leaves, and is increased by additions to the point of 
its axis, or by simple elongation or dilatation; this kind of stem 
is called acrogenous. 
In the stems of exogenous plants the pith is almost exclu¬ 
sively composed of cellular tissue, though, in some few instances, 
De Candolle and Mirbel have discovered woody fibres; and also 
in Nepenthes a considerable quantity of spiral vessels have been 
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