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84
THE MARINE ALGÆ OF NEW ENGLAND.
Var. DENSA.
Fronds six to eight inches long, main axis densely clothed with very numerous short
branches.
Common on stones near low-water mark along the whole coast.
The var. densa at Gloucester, Mass., Mrs. Davis.
A common species, recognized by its tough, somewhat elastic substance, and reminding one of bunches
of small leather shoe-strings. When soaked in water it gives out a large amount of slime, and is not
easily mounted. To the naked eye it resembles some of the forms of Dictyosiphon, but the microscopic
structure is very different. The variety has been collected several times at Gloucester, but has not been
received from other localities.
MESOGLOIA, Ag.
(From µεσος [mesos], the middle, and γλοιος [gloios], slimy.)
Fronds olive-brown, gelatinous, filiform, branching; axial layer composed of
filaments rather loosely united into a solid mass, which soon becomes fistulose;
peripheral layer of short horizontal filaments, packed in a gelatinous substance;
unilocular sporangia oval, borne at the base of peripheral filaments; plurilocular
sporangia unknown.
The old genus Mesogloia has been divided by modern algologists into a number of genera. In the
present instance we have kept in Mesogloia the species in which the peripheral filaments are not
transformed into plurilocular sporangia, and have placed in Castagnea the species in which they are so
transformed. The distinction between Mesogloia and Castagnea is artificial, because the plurilocular
sporangia of Mesogloia proper are unknown, and it is not impossible that they may be formed from the
peripheral filaments themselves, as in Castagnea. The development of the fronds is not well known,
and the genera founded upon the variations in the mature fronds in the present group are plainly
artificial. As regards its development, M. divaricata resembles very closely C. virescens. From a disklike
expansion, composed of a single layer of cells, which form spots on the substance upon which it is
growing, arise vertical filaments, which end in a hair such as is found in Ectocarpus and other
Phæosporeæ. The vertical filaments produce, usually only on one side, fasciculated branches terminated
by a hair, beneath which is a cluster of short moniliform filaments. Besides these there arise, at a later
period, rhizoidal filaments. The mature fronds of the two species above named may be regarded as a
collection of filaments with a trichothallic growth, which have become twisted together and partially
united by means of the rhizoidal filaments, and whose fasciculated branches constitute what, in the
mature plant, seems to be a distinct cortical layer. In Castagnea virescens the separate filaments, with
their lateral fasciculate branches, can easily be isolated by dissecting the smaller branches, and the
same thing can also be accomplished with Chordaria divaricata, although not so easily. The species of
Mesogloia and Castagnea should not be dried under too heavy pressure, and alcoholic specimens are
much better for study than those mounted on paper.
M. DIVARICATA, Kütz. (Chordaria divaricata, Ag.; Phyc. Brit., Pl. 17; Ner. Am. Bor.,
Vol. I, Pl. 11 a.)
Fronds tufted, lubricous, six inches to two feet long, branching very irregular,
generally without a definite main axis; branches flexuous, ultimate branches very
numerous, short, and divaricate, at first solid,