Part 176. The red algae (Class Rhodophyceae), the largest and one of the most highly developed groups of seaweeds, are characterized by the presence of a red pigment called phycoerythrin,1 which very generally masks the chlorophyll completely. The carrageen already studied belongs to this class.

1 Phy"co-er'y-thrin - Gr. erythros, red.

A comparatively simple type is the thread-weed (Nemalion, Fig. 320). The thallus is small and with slender branches which grow at the apex but do not show much differentiation among the vegetative cells, Male gametangia (I, sp) are developed at the tips of certain branches, and these emit minute, spherical gametes having no flagella or other means of locomotion. At the tips of other branches female gametes appear, each in the form of a flask-shaped cell with long, slender neck (I, t). Fertilization is effected by fusion of male gametes with the projecting end of a female gamete to which the little spheres have been brought by currents in the water. After fertilization the basal part (7-7, c) gives rise to several branches, each of which finally produces at its tip a spheroidal spore that soon separates, and attaching itself to some support, developes into a new Nemalion plant. Spores which are thus the indirect product of fertilization are called carpospores.2

2 Carp'o-spore - Gr. karpos, fruit.

Fig. 320. Thread weed (Nemalion multifidum. Thread weed Family, Helminthocladiacece). I, branch bearing male gametangia (sp), and a female gametangium with swollen base (c) and slender neck (t). II, a female branch after fertilization, the neck (t) withered, and the base beginning to divide for the formation of branches. III V, later stages in the formation of branches which finally bear spores. All much magnified. (Thuret and Bornet.) A brownish red seaweed with branches 5 20 cm. long, on exposed rocks at low water mark, North Atlantic coast.

Fig. 320.-Thread-weed (Nemalion multifidum. Thread-weed Family, Helminthocladiacece). I, branch bearing male gametangia (sp), and a female gametangium with swollen base (c) and slender neck (t). II, a female branch after fertilization, the neck (t) withered, and the base beginning to divide for the formation of branches. III-V, later stages in the formation of branches which finally bear spores. All much magnified. (Thuret and Bornet.)-A brownish-red seaweed with branches 5-20 cm. long, on exposed rocks at low-water mark, North Atlantic coast.

Sexual reproduction in Chondrus (Fig. 118) as also in almost all of the Rhodophyceae is by means of carpospores. The process is often much more indirect and complicated than in Nemalion; and, as is the case with Chondrus, the details may be somewhat modified by formation of the carpospores within the thallus, as shown in Fig. 321. Non-sexual reproduction is accomplished very generally throughout the class by non-motile spores, which are produced usually four in a sporangium. The sporangia may be either at the surface or embedded within the thallus as in Chondrus.

Fig. 321. Carrageen (see also Fig. 118). A, transverse section through a fruiting branch showing the spore clusters embedded in the thallus, 60/1. B, same, 270/1, showing rind (r), pith like interior (m), and spore clusters (s).

Fig. 321.-Carrageen (see also Fig. 118). A, transverse section through a fruiting branch showing the spore-clusters embedded in the thallus, 60/1. B, same, 270/1, showing rind (r), pith-like interior (m), and spore-clusters (s). (Luerssen.)

The thallus in this genus, as with a large part of the class, exhibits a differentiation of cells similar to that already described in our examples of brown algae, and many of the red seaweeds rival the brown in elaborate forms of thallus simulating remarkably the shoots of higher plants.