ANATOMY OF TISSUES. 101 
consists mainly of stem bundles, the leaf-traces being attached 
to these. There is still a third method of arrangement found 
in stems of certain ferns with many rows of leaves.- Such 
plants show on a cross-section of stem several concentric bundle 
rings, each of which forms a net-like cylinder as before described. 
On a long section these several hollow cylinders have the ap- 
pearance of conical mantles widening toward the apex of the 
stem. There are numerous places of connection between these 
mantles, caused by the anastomosing of the leaf-trace bundles, 
making the actual course of development extremely difficult to 
discern.? 
In several instances the bundles of fern-stems are collateral 
instead of concentric; for example, Osmunda. <A secondary 
epidermis or periderm is never developed on the fern stem, but 
frequently its place is supplied by a well developed hypoderma. 
BRANCHING OF STEM. 
The anatomy of the branches, or secondary axes as they are 
sometimes called, is similar to that of the primary stem. The 
exact place ot origin and the first stages of development of 
these side axes is often very difficult to determine. In the 
Pteridophytes there is much uncertainty respecting the manner 
of branching in the various classes. Some few facts of general 
application have been ascertained, but before stating these, a 
brief description of the morphological relations of stem and 
branches may be inserted. 
There are two principal systems of branching, the mono- 
podial and the dichotomous. In the monopodial system, the 
main axis continues to prolong itself, its meristems remaining 
active, while the side branches rise from a new cluster of initial 
cells, near the apex in acropetalous order. All these new 
branches are of the same order or rank, in respect to the 
1 For a more detailed account see De Bary’s Comparative Anatomy. 
