INTRODUCTION. 
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in all arrangements of British plants been associated with Pilularia, on account of 
their both being water plants, both having round and filiform leaves, and bearing 
two kinds of grains or capsules ; but, except in these particulars, they are totally 
different from each other, and Isoetes is really more closely related to the 
Lycopodiaceae. [The roots of Isoetes are tufted, composed of round, smooth 
branched fibres ; its leaves grow from the crown of a bulb-like, perennial stem, 
which increases in size by annual layers of fibro-vascular structure. The roots 
emerge from this, below, like those of bulbs and cornes, but have this peculiarity, 
that the outermost are the oldest, the new ones springing from the centre like the 
leaves in a bud. — A. II.] The leaves consist of four hollow tubes, united together, 
but so brittle are they, that the cells are often broken into each other by the pressure 
used in drying the plants, and therefore the leaf generally appears like a single 
tube, divided into cells by transverse dissepiments. It is expanded at the base so 
that the joint or cell next the stem becomes a receptacle for the fruit, which is of 
two kinds ; one minute, like pollen-grains, in the inner leaves — the other, much 
larger, confined to those on the outside of the plant. [These relative positions arise 
from a kind of periodicity in the growth. — A. II.] The spores are set free by the 
decay of the walls of the receptacles. The leaves are said to have stomata, and to be 
circulate in vernation, but neither of these is the case. Being a submerged water 
plant, of course it is without stomata, and Martius expressly says, “ vernation not 
circinate, but only a little bent.” An observation confirmed to me by four or five 
botanists of eminence. 
[Spores and Germination. As in the Ferns and their other Allies, the mode 
of reproduction of Isoetes has only been cleared up recently. The smaller 
spores do not differ importantly from those of the Ferns ; they are rather smooth, 
and of the shape of a quarter of a globe, and resemble pollen-grains in structure. 
The larger spores, which are produced in smaller numbers, are especially dis- 
tinguished by their size, the globular form when mature, the firm outer coat 
composed of three layers of membrane, and the presence of three elevated ridges 
radiating from a point upon the external surface. 
When these spores are discharged from the sporanges, they undergo very dis- 
similar courses of development. The small spores, after about a month, burst, and 
set free two or four minute vesicles which have been formed in their interior ; 
these, again, give birth to one or two lenticular vesicles, from each of which 
escapes a ciliated spiral filament, such as we have described in the preceding families. 
The large spores contain only oily and mucilaginous matter when first set free. 
If examined by the microscope in a few weeks after, they are found to be filled 
up by cellular tissue. The external firm coat is then soon ruptured, in the lines 
of the three convergent projecting ridges, so that three triangular free teeth are 
formed, which turn hack and leave a small portion of the internal cellular tissue 
bare. Upon this bare portion, at first in the centre, exactly under the point where 
the three ridges met, an arcliegonium is formed, exceedingly small in size, but 
essentially resembling those of the Ferns and Equisetacete. If the first is not 
fertilized, others are produced, and as many as eight have been observed. A 
