THE GENUS ISOETES IN NORTH AMEEIOA. 455 
in the amphibious and thickest in the terrestrial species, consisting in the former often of only 2—4, in 
the latter of 6-9 layers of parenchymatous cells; the median dissepiment is generally a little thicker 
than the transverse one. The anterior Jacwne are mostly somewhat larger than the posterior ones. 
e epidermis of the leaf consists of rectangular cells, mostly much longer than they are wide; 
only in J. pygmea are they comparatively short, and sometimes even square. Ina few species 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 
avery important character for the diagnosis and classification of the species. It was formerly 
thought that the submerged 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 iniaaed species with stomata and emerged ones 
without them, and we have one submerged species (J. echinospora) in which the typical 
European form is destitute of stomata, while the American varieties show either a few [366 (9)] 
stomata, often difficult to discover, or, rarely, numerous ones; so that in this interesting 
species the question arises whether the presence or absence of stomata alone can specifically separate 
forms otherwise scarcely distinguishable, as that acute observer of these plants, A. Braun, has main- 
tained, or whether the stomata do not always play that important part in classification generally 
assigned to them. Below will be found directions for the investigation of the stomata 
The parenchyma of the leaf consists of a few or several layers of ch leceusterll-baaring cells, 
(1) under the epidermis, (2) around the central bundle of vessels, and (3) forming the dissepiments, 
which cross each other in the centre of the leaf. 
An important element in the leaf structure is found in the peripheral bast-bundles, which are 
present in some and absent in other species; and their presence often, but not always, coincides with 
the presence of stomata. When present they commonly form four bundles, two in the two anterior 
angles of the leaf, and two where the median dissepiments connect with the anterior and the pos- 
terior wall of the leaf; in J. Nuttalliz I find only three bundles, the anterior median one being 
wanting; in J. Cubana six bundles are visible, the two additional ones being located where the 
transverse dissepiment unites with the outer wall. In some rigid-leaved land species, e. g. J. mela- 
nopoda, often several smaller accessory bundles are found scattered under the epidermis. 
The examination of the fresh Jsoétes leaf is not very difficult; particles of the epidermis are 
easily removed, and show the stomata, when present, very distinctly. Where there are few stomata, 
the epidermis from different parts of the leaf must be examined, and especially from the tip, as they 
are more apt to be found there. In dried specimens the leaf must be soaked, the alge which often 
adhere to the surface have to be carefully scraped off, after which I make several sections } or $ 
line wide, lay them open by a vertical slit, detach the central bundle, and then scrape very gently 
the inner surface so as to remove the parenchymatous cells which obscure the appearance of the 
stomata. This process can be aided by an immersion of the specimen in a weak solution of caustic 
potash. The work is often a difficult one when the specimen is very old or poorly pre 
served, and requires a good deal of patience. Sometimes the application of iodine will [867 (10)] 
very distinctly show the stomata by coloring their guard-cells blue when only these con- 
tain amylon, but of course not when the other cells are also filled with that substance. A mag- 
nifying power of 150 to 250 diameters is best adapted to well exhibit the stomata. 
To find the bast-cells it is necessary to make the thinnest possible transverse sections of the 
leaf, boil them well, and, if they do not then show under water as bundles of minute, thick-walled 
darkish cells close to the epidermis, very distinct from the much larger epidermis cells, the appli- 
cation of a solution of caustic potash, to clear the preparation, will readily bring them out. The 
same magnifying power which we use for the examination of the stomata may be applied for the 
study of the bast-bundles. 
