ANATOMY OF TISSUES. a a he 
is either one-sided forming a sympodium, or the two arms join 
so as to make a network. (Figs. 49 and 50.) 
We may now compare the two types, the mono- and the 
dicotyledonous in respect to the chief differences in the course 
of their bundles. In the latter, the course of the bundle is 
radially straight, but tangentially oblique ; while in the mono- 
cotyledon it is just the opposite, it is radially oblique and 
tangentially straight. To illustrate this, successive cross-sec- 
tions of the dicotyledonous stem present the same appearance, 
a circle divided into three parts, pith, bundle-zone, and rind- 
zone. If we wish to represent the course of the several bun- 
dles, we must consider a whole or a part of the hollow cylinder 
which they compose as taken out of the stem and flattened on 
a plane surface. We shall then have such a figure as 49 or 50, — 
the lines in Fig. 49 representing the course of the leaf traces 
with a single strand, and in Fig. 50 that of those with divided 
strands. 
If we wish to show the course of the bundles in the mono- 
cotyledonous stem, we must cut a radial section from the centre 
to the circumference, and that will contain the bundle from its 
entrance into the stem to its point of intersection with the older 
bundle, or till it ends free. Of course, reference is made here 
only to those stems which may be considered as following the 
normal type. There are many plants which vary from this. 
For example, the tangentially oblique course of the leaf-traces 
of dicotyledons is not common to all. In many stems they run 
nearly parallel with the long axis, bending only at the nodes. 
This is usually the case in stems with opposite leaves. It may 
also be mentioned here that when their course is tangentially 
oblique, they seldom continue long bending in the same direc- 
tion so as to form a spiral, but more often they alternate from 
right to left, thus taking an S-formed course. In a very few 
dicotyledonous stems the bundle system is so arranged that the 
course of the stem bundles is radially oblique, like that of the 
monocotyledons. 
