Order 1. =Botrydiaceæ.= The thallus in the vegetative condition is unicellular, club-shaped, with a small single (_Codiolum_) or repeatedly dichotomously branched system of colourless rhizoids (_Botrydium_, Fig. 56 _a_), by which it is attached to objects immersed in salt water (_Codiolum_) or to damp clay soil (_Botrydium_). Asexual reproduction by zoospores with one (_Botrydium_) or two cilia, and by aplanospores. The sexual reproduction is only known in _Botrydium_, and takes place in the following manner: in the part of the thallus which is above ground and in an active vegetative condition, several round cells (Fig. 56 _c_) are formed, which may be green or red according as they grow under water, or exposed to the strong light of the sun. These cells must be considered as “gametangia” as they produce many gametes (_d_) provided with two cilia. The zygote (_h_, _i_) formed by the conjugation (_e_, _f_, _g_) may either germinate immediately, or become a thick-walled resting-cell of an irregular, angular form. Order 2. =Bryopsidaceæ.= The thallus in the vegetative condition is unicellular, and consists at the lower extremity of branched rhizoids, while the upper portion is prolonged into a stem-like structure of unlimited growth, producing, acropetally, branches and leaf-like structures. The latter have limited growth, and are separated by a cross wall from the stem, and become gametangia, or drop off. The gametes have two cilia, and are of two kinds: the female, which are green and large and the male, which are of brownish colour and smaller. Zoospores or any other method of asexual reproduction are unknown. Only one genus, _Bryopsis_, living in salt water. [Illustration: FIG. 57.--_Bryopsis plumosa_. A the plant, natural size. B A portion (enlarged) which shows the growing point (v), and the leaves derived from it in acropetal succession.] Order 3. =Derbesiaceæ.= Only one genus, _Derbesia_, living in saltwater. The zoospores, which are formed in a few lateral, swollen zoosporangia, possess one nucleus which has arisen through the coalescence of several, and they resemble the zoospores of _Œdogonium_ by having a circle of cilia attached at the base of the colourless spot. Order 4. =Vaucheriaceæ.= The thallus consists, in the vegetative condition, of a single irregularly or dichotomously branched cell, without differentiation into stem or leaf; root-like organs of attachment may however occur. Asexual reproduction by zoospores, which are formed singly in the extremity of a branch cut off by a transverse wall. They contain many nuclei, and bear small cilia situated in pairs, which give the appearance of a fine “pile” covering the whole or a great part of the surface. Akinetes, aplanospores, and phytoamœbæ (naked masses of protoplasm, without cilia, which creep like an amœba on a substratum) may occur under certain conditions.