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20
THE MARINE ALGÆ OF NEW ENGLAND.
normal condition. When, however, they grow in positions where they are much
exposed to the light they become green, and in decaying they pass through various
shades of orange and yellow to green. Their favorite place of growth is below lowwater
mark and in deeper water, but some species grow in tide-pools. The fronds
vary in structure in the different genera, but as a rule they are less complicated than
the fronds of the Fuci and Laminarieæ. The non-sexual mode of growth is by means
of bodies called tetraspores, formed by the division of a single cell into four parts.
The divisions may be at right angles to one another, when the tetraspore is said to
be cruciate; they may be parallel to each other, in which case the tetraspore is said
to be zonate; or they may be arranged as in Pl. XI, Fig. 1 a, when it is said to be
tripartite. The tetraspores may either be isolated or collected in wart-like masses,
called nemathecia. The individuals which bear the tetraspores are, with rare
exceptions, distinct from those which bear the sexual fruit or cystocarps.
Occasionally both kinds are found on the same individual, as sometimes happens in
Callithamnion Baileyi and Spyridia filamentosa. The tetrasporic plants, taking the
order as a whole, are decidedly more abundant than those which bear the
cystocarps. The sexual fruit, called the cystocarp, is formed by the action of
antherozoids upon a structure called the trichogyne, which forms a part of the
procarpe. The antherozoids are small colorless spheres, destitute of cilia. They are
borne singly in cells, which are agglomerated in various forms, which differ in the
different genera, but are usually either in the shape of short, dense tufts, or else are
siliculose in outline. In Chondria the antheridia cover the surface of irregular disklike
branches, and in membranous genera they form spots on the surface.
The name of procarpe was given by Bornet and Thuret to the collection of different
cells, of which the female organ is composed before fertilization. The procarpes are
borne on the younger parts of the frond generally near the surface. The cells of
which they are composed may be divided into two sets—those which take part in the
act of fertilization and those from which the spores are formed. The former consists
of the trichogyne, a long, slender, hyaline hair, at whose base is the trichophore. The
latter set, called by Thuret and Bornet the carpogenic cell or system, varies in the
different genera, and is in most cases too complicated to be explained in the present
article. In the simplest genera, as in Nemalion and Batrachospermum, the
antherozoids come in contact with the extremity of the trichogyne, where they
remain fixed for a considerable time. The contents of the antherozoid, or
antherozoids—for more than one may be attached to the trichogyne—pass into the
trichogyne, and, in consequence of this action, a change takes place in the
trichophore, which divides, the divisions growing into short filaments, which are
formed into chains of spores by transverse divisions. In this case the trichophore
represents the carpogenic cell. In Nemalion the cystocarpic fruit is a globular mass
of spores, arranged in filaments