456 THE GENUS ISOETES IN. NORTH AMERICA. 
I would advise any one who desires to study the structure of Jsoétes leaves to commence with 
well known species and good (if possible, fresh) specimens, and make himself familiar with the 
manipulation and with the appearance of their baadia under the microscope before he proceeds to 
study unknown and difficult specimens. 
The arrangement of the leaves in the species with two-lobed trunks is at first distichous, and 
in J. noddnensons it remains so through life; in all the others the leaves soon enter into a more 
complicated phyllotactic order; in the larger ones, with many leaves the 33 and even the 2} order 
is found. 
The number of leaves varies from 5 or 10 (L. pygmeea, I. melanospora) to 100 or even 200 (J, 
Engelmanni, var. valida), and their length from 3-1 inch (in J. pygmea) to 1 or 2 feet (in some 
forms of J. flaccida and I. Engelmanni) ; their color from light and fresh yellowish-green (J. Lngel- 
mannt) to dark and dull green (J. lacustris); their rigidity is greatest in the terrestrial species, and 
also in some submerged ones; and least in most amphibious species, which often float their leaves 
on the surface of the receding water, or in some submerged ones, the leaves of which, taken out of 
the water, collapse like the soft hair of a wet pencil. The submerged species vegetate and retain 
their verdure throughout the winter (whence, it is said, the name of the genus is deriv 
Isoétes, equal at all seasons), but the others lose their leaves soon after their fuente ti in [868 (11)]} 
autumn, some of our terrestrial ones even already in summer. 
The broad membranaceous sheathing base of the leaf is without air cavities, stomata, or bast- 
bundles ; in sterile leaves it gradually contracts into the leaf itself. Those leaves are usually sterile 
which develop at the beginning and the end of the season. The fertile leaves have in their base 
an excavation which bears the spore-case, sporangium, adnate with its back to the midrib. Above 
this excavation, and separated from it by a deep transverse depression or slit, we find a stipule-like 
organ of triangular more or less elongated shape, cordate at base, appressed to the leaf, which is 
termed the digula ; it is small, and in not very fresh leaves often mutilated and difficult to make 
out. The morphology of these parts is obscure, and their diagnostic value not great. 
The sporangium is oblong or circular (both forms often seen in the same species), from 4 to 1 
line long in J. melanopoda ; 1 to 2 lines in L. pygmeea, Tuckermant, echinospora, and saccharata ; 14 
to 2} lines in J. lacustris, Bolanderi, and flaccida ; often a little larger in J. Butleri and Nuttallii ; 
2 to 4 lines in L. riparia, Engelmanni, melanopoda, and Cubana; and in larger forms of L. Engel- 
manni I have seen it 8 to 9 lines long. It is somewhat flattened, and often slightly concave on the 
ventral side; it is entirely naked, or (the usual case) it is on its sides and principally upwards 
partially covered by a fold of the ventral side of the leaf-base, the veil (velum); in a few species 
(Z. flaccida, melanopoda, and Nuttallii) this fold extends over the whole sporangium, completely 
covering it (velum completum). The sac of the sporangium is composed of two layers of cells; the 
outer, epidermidal layer consists of elongated, often variously bent or hooked and curiously inter- 
laced cells, mostly thin-walled and transparent ; in some species (e.g. J. riparia, I. saccharata, 
I. melanopoda) groups of brown, thick-walled, (so-called) ssletenphym-cells are mixed with the 
transparent ones, giving the spore-case a dotted appearance visible even to the naked eye. The 
spore-case is traversed by numerous parallel strings. 
Some sporangia, called macrosporangia, contain larger or female spores (macrospores or gynospores) ; 
others are filled with the minute or male spores (microspores or androspores); these 
are called microsporangia. Almost all the species are moncecious, bearing macrosporangia [369 (12) ] 
on the base of the outer, and microsporangia on that of the inner leaves. I am not aware 
that any exotic species behave differently, but here we have two species which deviate from this 
norm. J. melanopoda in Illinois as well as in the Indian Territory, from both of which localities 
I have examined several hundreds of specimens, is polygamous, i.e. moncecious as well as diccious, 
and shows about an equal number of male, female, and moncecious plants. The allied L Buélert 
