pressure, as the form and direction o f the branches and leaves
can thus be better seen.
Between fifty and sixty species o f Sphagnum are known, of
which about one-third are tropical ; but they are most abundant
in the north and south temperate zones, in the higher latitudes
of which they cover a large expanse of surface.
Among the exotic species we may mention S . sericeum,
C. M ü l l ., S. Holleanum, S. Junghuhnii, and 6’. Gedeanum, D ozy
et M o l k ., peculiar to the Eastern Archipelago, the two former only
known in a barren state, but remarkable in having the stem leaves
precisely like the branch leaves in form and structure, their hyaline
cells being without fibres, but with a single apical pore.
Brazil is rich in forms, comprising S. erythrocalyx, perichoetiale
subrigidum, sparsum, and gracilescens o f H ampe, Caldense and pul-
chricoma, C. M ü l l . ; and in Central America we find S. Negrense,
limbatum, Mexicanum, and Peruvianum o f M it t en , Meridense,
C. M ü l l ., longifolium, M andón, subcuspidatum, S chimp. From
Guadaloupe, .S'. Antillarum, C. M ü l l ., Guadalupense, Herminieri,
and Husnoti, S chimp. ; and from Australia and N ew Zealand,
.S'. Mossmanni and cymbifolioides of C. M ü l l ., Australe, molli-
culum, antarcticum, confer turn, and Novo-Zealandicum o f M it t e n .
T h e only species from Tropical Africa is .S. Africanum, D u by .
It would be desirable that all the exotic species should be submitted
to a careful investigation and figured, although it would
probably result in reducing their number, for each author appears
to have set up a different standard in the characters regarded as
specific.
W ith respect to the great variability in some species of
Sphagnum, it is difficult to decide on any determining cause : soil
certainly does not appear to exert any controlling influence, for
we may often see several varieties growing together in the same
locality, but each preserving its proper character. T h e difference
in the seasons, as regards moisture or drought, is probably an
important factor in the question o f development, size, or density ;
but the colour must be the result o f vital action, since it depends
on the colour o f the chlorophyl deposited in the narrow cells of
the leaves, which, shining through the transparent hyaline cells
while moist, gives the beautiful and varied tints to the Sphagnum
tufts while growing in their native habitats, and so imperfectly
retained in dried specimens. Probably in all Sphagna we find
a tendency to va ry in two directions ; the first and most marked
o f these is to form a compact or dense variety, by the shortening o f
the internodes, and consequent close approximation o f the branch
fascicles; this is well seen in such states as 5 '. A u s t in i var. imbri-
catum, S . papillosum var. confer turn, S . cymbifolium var. congestum,
S. rigidum var. compactum, and acutifolium var. arctum. T h e
second is that in which the leaves o f the branches which are
normally straight and imbricated, exhibit a greater or less inclination
to become squarrose ; this does not render the plants so
conspicuous as the last state, and is sometimes so slight as not
to require indication by a special name ; and even in cases where
it may be quite evident, as in cymbifolium var. squarrosulum,
we find intermediate forms which completely connect it with the
type ; nay, sometimes in the same tuft we may observe the leaves
on plants at the circumference distinctly tending to squarrose,
while those in the centre o f the patch do not exhibit it at a l l ;
in this w ay it is quite possible to pick out the links that join
S. teres to S. squarrosum.
In none do we find the range o f variability so extensive as
in .S', acutifolium, and considerable experience is needed to identify
satisfactorily all the forms o f this protean plant. On this
account we must, I think, rely on histological distinctions for the
essential character o f species, as size, colour, direction o f leaves,
habit, and presence or absence o f fibres in the hyaline cells o f the
stem leaves, alike fail in this species to afford any stable grounds
for the establishment o f new ones, and the same considerations
must guide us in dealing with the r e s t ; they will thus stand on
firm ground, and be more satisfactory to the true botanist than
the most extensive series o f phantom species.
T h e structures we must look upon as o f most importance in
affording specific distinctions are—
I. T h e number o f cell strata composing the cuticle which
invests the stem o f the plant.
T h e stem leaves, with respect to their form, relation o f the
two kinds o f cells, and structure o f the apex.
T h e branch leaves, as to form, involution o f margin, apex,
relative position o f the hyaline and chlorophyllose cells,
and presence or absence o f papillie on their internal
lateral walls, and also the presence and size o f the foramina.
T h e number and direction o f the branches in
each fascicle, and nature o f the retort cells o f their
cuticle, are also to be taken notice of.
2 .