KINDS AND FORMS OF LEAVES. 
43 
Exogenous Stems. 
of Flax-stem magnified; and also in Fig. 81, which shows the same structure in 
a woody stem, namely, in a shoot of Maple of a year old, cut both crosswise and 
lengthwise. 
116. The difference becomes still more 
marked in stems more than one year old. 
During the second year a new layer of 
wood is formed outside of the first one, 
between it and the bark; the third year, 
another layer outside of the second, and so 
on, a new layer being formed each year 
outside of that of the year before. The 
increase is all on the surface, and buries the older wood deeper and deeper in the 
trunk. For this reason such stems are said to be exogenous or outside-growing 
(from two Greek words which mean just this), a new layer being added to the wood 
on the outside each year as long as the tree or shrub lives. And so the oldest wood, 
or Heart-wood, is always in the centre, and the newest and freshest, the Sap-wood, 
at the circumference, just beneath the bark. 
117. The heart-wood is dead, or soon becomes so. The sap-wood is the only 
active part; and this, with the inner bark, which is renewed from its inner face every 
year, is all of the trunk that is concerned in the life and growth of the tree. 
118. Plants with exogenous or outside-growing stems, especially those that live 
year after year, almost always branch freely. All common shrubs and trees of 
the exogenous class make a new set of branches every year, and so present an ap¬ 
pearance very different from that of most of those of the endogenous or inside¬ 
growing class. 
§ 3. Of Leaves. 
119. Leaves exhibit an almost endless variety of forms in different plants; and 
their forms afford easy marks for distinguishing one species from another. So the 
different shapes of leaves are classified and named veiy particularly, — which is 
a great convenience in describing plants, as it enables a botanist to give a correct 
idea of almost any leaf in one or two words. We proceed to notice some of the 
principal kinds. 
120. Their Parts, A leaf with all its parts complete has a Blade, a Footstalk, 
and a pair of Stipules at the base of the footstalk. Fig. 82 shows all three parts 
