
INTRODUCTION. nt 
The vessels, Mr. Berkeley continues, though varying greatly in size, 
are accompanied by cellular tissue, and surrounded by elongated 
pale wood cells (pleurenchym), beyond which is parenchym, mixed 
with resinous cysts. The hard coat which encloses the vessels 
with the pleurenchym and parenchym, belongs to the general mass 
of cellular tissue, and not to the wood, which is represented by the 
minute quantity of pleurenchym. 
The structure of other Ferns corresponds more or less closely with 
this, the chief difference being in the disposition of the bundles 
and of the hard attendant tissue, which is infinitely varied. Thus 
in Pteris aquilina, the hard tissue which in the tree Ferns encloses 
the principal vascular bundles, is disposed in two curves on either 
side of which the vascular bundles are arranged, each enclosed by 
a thin coating of tissue denser than the general mass. Sometimes 
these two ares meet at one extremity, sometimes at both, especially 
when a stipes is given off. Besides these two main masses of dense 
tissue, there are many scattered fibres. In other cases, this peculiar 
- dense tissue is altogether scattered about in little fibres, like those 
of Pteris aquilina just mentioned ; in some, as in Oleandra hirtella, 
it is either converted into or replaced by curious cysts which display 
a fibrous structure. In many cases, as in Pleopeltis leiorhiza, where 
the bundles are arranged in one principal circle with a few out- 
liers, it appears to be entirely wanting. In this, and many other 
Ferns there.appears to be scarcely any distinct cortical layer, the 
transition to the general mass of cellular tissue being almost 
imperceptible.* i 
The same kinds of tissues are found in the stipites and rachides, 
where there occur detached rounded bundles, or flattened, often 
curved, plates of the harder tissues, in the midst of the cellular 
mass. The arrangement and number of these bundles and plates 
* Berkeley, Introduction to Cryptogamic Botany, 515. 
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