Family 3. Siphoneæ.
The thallus has apical growth, and in the vegetative condition consists generally of one single (in the Valoniaceæ most frequently of more) multinuclear cell, which may be much branched, and whose separate parts in the higher forms (e.g. Bryopsis, Fig. [57]; Caulerpa, Fig. [59], etc.) may be differentiated to perform the various physiological functions (as root, stem and leaf). Vegetative multiplication by detached portions of the thallus (gemmæ); asexual reproduction by zoospores, akinetes, or aplanospores. Sexual reproduction by gamete-conjugation, rarely by oogamous fertilisation. The zygote or oospore germinates as a rule without any resting-stage.
Fig. 56.—Botrydium granulatum: a an entire plant forming swarmspores; b swamspores; c an individual with gametangia; d, gamete; e, f, g conjugation; h zygote seen from above; i the same in a lateral view.
Most of the Siphoneæ occur in salt water or on damp soil. Many (e.g. Dasycladaceæ) are very much incrusted with lime, and occur, in the fossilized condition, in the deposits from the Cretaceous period to the present time. The Siphoneæ are connected by their lowest forms (Botrydiaceæ or Valonia) with the Protococcaceæ, but show also, through the Valoniaceæ, points of relationship to the Cladophoraceæ.
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.
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.