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I
in advance of the one from which it emanated in an incipient
condition; hence in the Peritrieheae, starting with the genus
Tubulina, the marked characteristic of the order—a perforated
sporangial wall—evolves gradually through Protodermium to
Pktydium, where it attains its highest phase of development,
and then terminates abruptly; whereas’ in Pnteridium, a comparatively
primitive genus, we get a transition from slightly
perforated walls to a complete breaking up of their substance
into an irregular network, which leads by degrees to the central
columella and capillitium characteristic of the Golumelliferae.
PERITRICHEAE.
The present order includes the most primitive types met with
in the Myxngastres, illustrated by such genera as Tubulina, of
which the plasmodiocarp condition, representing a plasmodium
covered by a common cortex, is the lowest phase; when the
substance of the plasmodiocarp becomes broken up into smaller
portions by special walls, an aethalium is produced, and finally,
by the gradual isolation of the components of an aethalium we
get the sporangium or most highly specialized form of reproductive
organ, characteristic of the higher sections; but we
everywhere meet with plasmodiocarp and aethalioid conditions,
proving the comparative instability of the acquired sporangial
phase. In the present order we learn that the primitive idea
in connection with spore dissemination was that of modifying,
or rather differentiating, the sporangial wall. The original
function of the wall was protective, as illustrated in the genus
Tubulina,, where we find this to be its only function, there being
no provision for spore diffusion other than that brought about
by the decay of the wall, which does not take place readily, and
the subsequent dispersion of the spores by wind or rain.
Passing on to such genera as Pnteridium and Clathroptychium,
we find the sporangial wall yet performing, as it continues to
do in every section, the function of protection, and remaining
intact until the spores are fully differentiated; but in these
genera certain portions of the wall have become specially
thickened, and at the period of maturity, the thickened portions
are persistent, while the remaining portions of the wall disappear,
and as this thickening is usually of a determinate
nature, and utilized as a generic factor, we meet with the small,
fairly regular openings in the lateral walls presented by one of
the members of the genus Pnteridium, or the still larger
openings of Clathroptychium ; but, although the modification is a
step in the right direction, owing to the fact that the species
constituting the two genera named retain the aethalioid condition,
the new idea is of but little practical value; and it is only
when we come to Gribraria, the components of which for the
most part have reached the condition of forming small detached
plasmodia, each only sufficiently large to form a single sporangium,
that we realize the value of the perforated sporangial
wall, which, other things being equal, is better than the sealed
up, entire sporangium, inasmuch as at the moment the spores
are mature, there is a chance of being removed from the
sporangium by wind or rain ; but in Gribraria the stem, although
usually elongated, is rigid, and it is in the genus Bietydium that
arrangements for spore diffusion from a perforated sporangium
is most perfect. In this genus the thickened, permanent
portions of the sporangium are arranged in the form of ribs
radiating from the base and converging again at the apex, being
connected by very thin, transverse bars, forming a hollow sphere
of lattioe-work ; this globe is pendulous from a long stem curved
at the apex, and while the stem is rigid during the young
condition, towards maturity the upper portion above the curve
becomes shrivelled and flaccid, and the sporangium hangs in
a condition ready to be dangled by the wind or the movements
of minute insects ; how effectual this method of spore diffusion
is in its way is proved by the specimens being generally found
destitute of spores. In Pnteridium, as already mentioned, we
find in one species the wall symmetrically perforated, whereas
another species has the walls of the component sporangia
forming the aethalium so irregularly perforated, and forming a
variable network, that at first sight the aethalioid nature of tlie
mass is masked, and this species may he considered as forming