Tuffen West, del. Edmund Evans.
Plate II.
Fig. 288.—Cutleria dichotoma. Section of lacinia of a frond, showing the stalked eight-chambered oosporanges growing on tufts with intercalated filaments. Magnified 50 diameters.
In Cutleria ([Fig. 288]) we have a special feature of interest with two kinds of organs, seemingly opposed to each other with regard to their reproductive functions. The sporangia not only differ from those of other species, but the frond consists of olive-coloured irregularly-divided flagella, on each side of which tufts (sori) consisting of the reproductive organs, intermixed with hair-like bodies, are scattered. The zoospores are divided by transverse partitions into four cavities, each of which is again bisected by a longitudinal median septum. When first thrown off they are in appearance so much like the spores of Puccinia that they may be mistaken for them, although so very much larger than those of other olive-coloured algæ.
Florideæ, the red algæ ([Plate II].), present many varieties of structure, although less appears to be known of their reproductive processes than of lower forms of cryptogamic plants. These are, however, of three kinds. The first, to which the term polyspore has been applied, is that of a gelatinous or membranous pericarp or conceptacle, in which an indefinite number of zoospores are contained. This organ may be either at the summit or base of a branch, or it may be concealed in or below the cortical layer of the stem. In some cases a number of spore-bearing filaments emanate from a kind of membrane at the base of a spheroidal cellular perisporangium, by the rupture of which the zoospores formed from the endochrome of the filaments make their escape. Other changes have been observed; however, they all agree in one particular, namely, that the zoospore is developed in the interior of a cell, the wall of which forms its perispore, and the internal protoplasmic membrane endochrome, the zoospore itself, for the escape of which the perispore opens out at its apex.
Fig. 289.—Dasya kutzingiana, with seed vessel and two rows of tetraspores. Magnified 50 diameters.
The second form is more simple, and consists of a globular or ovoid cell, containing a central granular mass; this ultimately divides into four quadrate-shaped spores; these, on attaining maturity, escape by rupture of the cell-wall. Another organ, called a tetraspore, takes its origin in the cortical layer. The tetraspores are arranged either in an isolated manner along the branches, or in numbers together; in some instances the branches that contain them are so modified in form they look like special organs, and have been called stichidia; as, for example, in Dasya ([Fig. 289]). Of the third kind of reproductive organ a difference of opinion exists as to the signification of their antheridia; although always produced in precisely the same situations as the tetraspores and polyspores, they are agglomerations of little colourless cells, either united in a bunch, as in Griffithsia, or enclosed in a transparent cylinder, as in Polysiphonia, or covering a kind of expanded disc of peculiar form, as in Laurencia. According to competent observers, the cells contain spermatozoids. Nägeli describes the spermatozoid as a spiral fibre, which, as it escapes, lengthens itself in the form of a screw. Thuret, on the contrary, says the contents are granular, and offer no trace of a spiral filament, but are expelled from the cells by a slow motion. The antheridia appear in their most simple form in Callithamnion ([Plate II]., Nos. 32 and 34), being reduced to a small mass of cells composed by numerous little bunches which are sessile on the bifurcations of the terminal branches. The spores are simpler structures than the tetraspores, and mostly occupy a more important position. They are not scattered through the frond, but grouped in definite masses, and generally enclosed in a special capsule or conceptacle, which may be mistaken for a tetraspore case. The simplest form of the spore fruit consists of spherical masses of spores attached to the wall of the frond, or imbedded in its substance, without a proper conceptacle; such a fruit is called a favellidium, and occurs in Halymenia; the same name is applied to the fruits of similar structures not perfectly immersed, as those of Gigartina, Gelidium, &c., where they form tubercular swellings on the lobes. In some, the tubercles present a pore at the summit, through which the spores emerge forth. In other cases, as in Ceramium ([Plate II]., Nos. 27 and 37), the spores occupy a more conspicuous place; a characteristic species is Delessaria ([Plate II]., No. 39), the coccidium either occurring on lateral branches, or is sessile on the face of the frond, when it consists of a case filled with angular-shaped spores attached to the wall of the case. The general external appearance of the red algæ is very varied, but it seems to attain to its deepest colouring in the Red Sea, which, it is said, is entirely due to the peculiarly vivid red seaweed. They are all exquisite objects for the microscope, as may be surmised from the few varieties presented in [Plate II]. The Florideæ of the warmer seas exhibit most elegantly formed fronds, as will be seen on reference to the “Phycologia Australica” of the late Dr. William Harvey, F.R.S.