ANATOMY OF TISSUES. 91 
described. We shall now have to return to the same figure 
to illustrate the manner of connection of leaf and stem by means 
of these bundles, which form the ground-work of support for 
the leaf as well as the stem. In the example before given, the 
bundles are described as forming in the stem and belonging to 
it. In this respect there are two classes of bundles, those which 
are formed in the stem and always remain there, growing in 
acropetalous order, and those which grow for some distance in 
the stem, and then curve outward into a leaf, so that they belong 
both to leaf and stem. The former are called, in German, 
stammeigene, or those peculiar to the stem, and they may al- 
ways remain entirely disconnected with any leaf bundle, or the 
latter may extend from the leaf base downward and attach itself 
to the stem bundle. Those bundles which belong to both stem 
and leaf are called leaf-trace bundles, and in respect to their 
place of origin and subsequent development they also form two 
classes. Those of one class are said to originate in the stem 
and bend out into the leaf; of the other, it is said that they 
originate at the base of the leaf and run in both directions, out- 
ward into the leaf blade, and downward into the stem until they 
reach the stem bundle and unite with it. 
In point of fact, the leaf originates in the stem and its base 
is in the stem when the bundle arises, but it is customary to 
describe the course of the bundles in this way. The stem, there- 
fore, may have leaf-trace bundles only, or stem bundles with 
separate leaf-trace bundles, or stem bundles with leaf-trace bun- 
dles attached. The leaf-trace bundle does not always enter the 
stem entire, but often divides into two or more strands which 
enter the stem at a node and run down through one or more 
internodes. Their subsequent course ,varies according to the 
varying species. 
Comparative Anatomy of the Leaf.— The simplest form of 
leaf has already been described, namely, a single plate of cells in 
the Bryophytes. In the leaves of all plants above the Bryo- 
