ANATOMY OF TISSUES. 109 
and then back toward the circumference, its course becoming 
more nearly parallel with the stem axis as it approaches the 
circumference. After running down through several inter- 
nodes, it either unites with the leaf-traces of older leaves, or 
ends free. The depth to which a leaf-trace penetrates the stem 
differs for the bundles of the same leaf. The midvein reaches 
farthest into the stem axis, those of the margin extend the 
least distance, while those remaining take an intermediate 
position. This bent course of the leaf-trace is generally accom- 
plished in a radial direction, that is, the leaf-trace lies in a 
plane parellel with, and cutting the long axis of the stem, but 
traversing this in an oblique direction; while in some excep- 
tional cases it does not lie in a single plane, but makes its way 
toward the axis by bending tangentially also. 
The stem bundles are often simple strands of bast or collen- 
chyma which usually appear near the circumference in the form 
of epidermal ribs. In some stems similar strands also occur 
near the centre. There are many variations from the type 
described, an important one being that of the Commelynaceae 
family which is similar to the dicotyledons in the arrangement 
of its vascular tissue. There is a circle of stem bundles lying 
near the periphery, at some distance from the leaf-trace bundles 
which are near the centre. The stems of Dioscorea and Tamus 
are the nearest to the dicotyledons, as here the leaf-traces are 
arranged in a single circle about the circumference. In their 
course, however, the bundles penetrate radially into the stem, 
extending to unequal depths, while those of the dicotyledons 
never approach the stem centre. | 
In certain long-lived monocotyledonous stems, growth in 
thickness is accomplished by means of a secondary meristem 
forming near the periphery of the stem. It differs from the 
so-called secondary growth of the dicotyledons, because the 
additional thickness is caused by the repeated intercalation of 
new bundles and their subsequent growth, while in the dicoty- 
