ENGELMANN-THE GENUS ISOETES IN N. AMERICA. [365 
no fresh spores are attainable. The root-fibres, sometimes longer 
than the leaves, are always dichotomously, and often many times, 
branched. — The upper, concave, surface of the trunk bears the 
leaves, the innermost or youngest ones often yet immersed in 
the trunk. 
The leaves are subulate or sometimes almost filiform tubular 
organs from abroad membranaceous sheathing base, mostly more 
or less quadrangular (broader and with sharper edges on the up¬ 
per or ventral, narrower on the dorsal side), or in our terrestrial 
species more triangular and keeled on the back. Their sheath¬ 
ing bases form the bulb , which can be compared to the bulb of 
liliaceous plants; in fertile specimens it is always larger and 
thicker than the trunk, and in some of the larger ones, e.g. /. 
Engelmanni and I. melanopoda , attains a diameter of one or two 
inches. The leaves above this base contain four longitudinal air 
cavities, lacunce , separated from one another by two dissepi¬ 
ments, a transverse and a median one, and irregularly divided by 
very thin transverse septa . The dissepiments are of different, 
pretty constant, thickness in the different species, thinnest in the 
amphibious and thickest in the terrestrial species, consisting in 
the former often of only 2 to 4, in the latter of 6 to 9 layers of 
parenchymatous cells; the median dissepiment is generally a 
little thicker than the transverse one. The anterior lacunce are 
mostly somewhat larger than the posterior ones. 
The epidermis of the leaf consists of rectangular cells, mostly 
much longer than they are wide; only in I. pygmcea are they 
comparatively short, and sometimes even square. In a few spe¬ 
cies the epidermis is entirely destitute of stomata, in the others it is 
pierced by stomata which communicate with the air-ducts, over 
which alone they are found. The presence or absence of stomata 
furnishes a very important character for the diagnosis and classi¬ 
fication of the species. It was formerly thought that the sub¬ 
merged species had no stomata, and those species which bear 
their leaves more or less exposed to the air were provided with 
them ; later discoveries, however, have shown that this rule does 
not always hold good, for we now know submerged species with 
stomata and emerged ones without them, and we have one sub¬ 
merged species (I. echinospora') in which the typical European 
form is destitute of stomata, while the American varieties show 
