NUTRITIVE ORGANS OF PLANTS. pO) 
appears in the branches, which spring from the azils! of leaves, every 
plant having its own peculiar and characteristic mode of branching, 
which, by giving diversity of aspect to trees, greatly contributes to the 
beauty of nature. Buds remain dormant during winter, the tender parts 
being protected from the frost by the scales with which they are covered. 
In spring, they begin to grow, when the sap ascends in the tree. Many 
buds, however, remain always dormant, especially on the lower parts of 
the stem and branches, the ‘strength of the plant being directed to the 
production of branches and leaves in its upper and outer parts; but if a 
branch is cut off over above a bud which it has thus seemed to neglect, 
the bud at once begins to grow, and to send forth a shoot in place of that 
which has been removed. Some: plants send forth branches of peculiar 
kinds. The runner of a strawberry is a kind of branch, although the 
plant is one of those in which the stem is scarcely developed. Such a 
branch, producing a bud at its extremity, serves for the extension of the 
plant over the surface of the ground, the bud soon sending down roots, 
and becoming capable of independent existence. 
Structure of Stems.—There are great differences in the structure of 
stems, and, according to these, plants are arranged into different classes, 
which differ also very much in other important characters. Three modes 
of structure are distinguished, and stems are accordingly described as 
exogenous, endogenous, or acrogenous, terms which are also applied to the 
plants to which they belong. 
Exogenous Stems.—Exogenous stems are those which have a bark distinct 
and separable from the wood, and in the centre of the stem a pith, which 
sends out branches, called medullary rays, 
through all parts even of the hardest wood. 
The medullary rays keep up a connection 
between the pith and the soft growing part 
immediately under the bark, and are essential 
to the nourishment of the wood. Exogenous 
stems increase in thickness by new layers of 
vessels formed under the bark, which gradually 
harden into wood; and the layers produced 
year after year are very often so distinctly 
marked, that the age of a tree may be pretty 
accurately computed by counting the number 
of them exhibited in a transverse section of 
its stem. All the trees and shrubs to be seen in Britain, and very 

Fig. 74.—Transverse Section 
of Exogenous Stem. 
1 The hollows where the branches unite with the stem, from Latin azilla, the armpit. 
2 These terms are derived from the Greek words gennad, to produce, exd, outwards, endon, 
inwards, and akros, highest ; and they are employed as indicating the manner in which these 
different kinds of stems grow. 3 From Latin medulla, pith. 
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