Fig. 10.—Sphæroplea annulina.
The highest form of the sexual reproduction is the Egg- or Oogamous fertilisation. The two coalescing cells are in the main unlike each other in form as well as size. The one which is considered as the male, and is known as the spermatozoid (antherozoid), developes as a rule in large numbers in each mother-cell (antheridium); they are often self-motile (except in the Florideæ, where they are named spermatia), and are many times smaller than the other kind, the female, which is known as the egg-cell, (oosphere). The egg-cell is always a motionless, spherical, primordial cell which can either float about freely in the water, as in the Fucaceæ (Fig. [9]), or is surrounded by a cell-wall (oogonium); generally only one oosphere is to be found in each oogonium, but several occur in Sphæroplea (Fig. [10]). The result of the spermatozoid coalescing with the egg-cell is, as in this case, the formation of a oospore, which generally undergoes a period of rest before germination (the Florideæ are an exception, a fruit-body, cystocarp, being produced as the result of coalescence).
An example of fertilisation is afforded by the Alga, Sphæroplea annulina (Fig. [10]). The filamentous thallus is formed of cylindrical cells with many vacuoles (r in A); some cells develope egg-cells (B), others spermatozoids (C), the latter in a particularly large number. The egg-cells are spherical, the spermatozoids of a club- or elongated pear-shape with two cilia at the front end (G; E is however a swarmspore). The spermatozoids escape from their cells through apertures in the wall (o in C) and enter through similar apertures (o in B) to the egg-cells. The colourless front end of the spermatozoid is united at first with the “receptive spot” of the egg-cell (see F), and afterwards completely coalesces with it. The result is the formation of a oospore with wart-like excrescences (D).
The female (parthenogenesis) or male (androgenesis) sexual cell may, sometimes without any preceding fertilisation, form a new individual (e.g. Ulothrix zonata, Cylindrocapsa, etc.).
Systematic division of the Algæ. The Algæ are divided into the following ten classes:
1. Syngeneticæ; 2. Dinoflagellata, or Peridinea; 3. Diatomaceæ; 4. Schizophyta, Fission-algæ; 5. Conjugatæ; 6. Chlorophyceæ, Green-algæ; 7. Characeæ, Stone-worts; 8. Phæophyceæ; 9. Dictyotales; 10. Rhodophyceæ.
Among the lowest forms of the Algæ, the Syngeneticæ, the Dinoflagellata, and the unicellular Volvocaceæ (Chlamydomoneæ), distinct transitional forms are found approaching the animal kingdom, which can be grouped as animals or plants according to their method of taking food or other characteristics. Only an artificial boundary can therefore be drawn between the animal and vegetable kingdoms. In the following pages only those forms which possess chromatophores, and have no mouth, will be considered as Algæ.
Class 1. Syngeneticæ.
The individuals are uni- or multicellular, free-swimming or motionless. The cells (which in the multicellular forms are loosely connected together, often only by mucilaginous envelopes) are naked or surrounded by a mucilaginous cell-wall, in which silica is never embedded. They contain one cell-nucleus, one or more pulsating vacuoles, and one to two band- or plate-like chromatophores with a brown or yellow colour, and sometimes a pyrenoid.