The Characeæ may be placed among the highest of the algals, if only for the complexity of their reproductive organs, which certainly offer a contrast in their simplicity of structure. Chara vulgaris, stonewort, is a simple fresh-water plant, preferring still freshwater ponds or slow-moving rivers running over a chalky soil. It thus derives the calcareous matter found in the axis of the plant, together with a small portion of silica. Its filaments (or branches, as some botanists prefer to call them) are given off in whorls. The Characeæ are a small family of acrogens, consisting of only two or three at most. They are monœcious and diœcious, the two kinds of fruit being often placed close together. They may easily be grown in a tall glass jar for observation. All that is necessary is to put the jar occasionally under the house tap and let the water run slowly over the top for a short time, thus renewing the contents without disturbing the plant. The hard water supplied to London suits chara better than softer water. Both chara and nitella are objects of great interest to microscopists, since in the former the important fact of vegetable circulation was first observed. A portion of the plant of the natural size is shown in [Fig. 290], No. 1.
Characeæ.
Fig. 290.—Diagrammatic sketch of Chara.
1. A stem of Chara vulgaris, natural size; 2. Magnified view (arrows indicating the course taken by the chlorophyll); 3. A limb, with buds protruding; 4. Portion of a leaf of Vallisneria spiralis, showing cyclosis of chlorophyll granules.
Each plant is composed of an assemblage of long tubiform cells placed end to end, with fixed intervals, around which the branchlets are disposed with great regularity. In nitella the stem and branches are composed of simple cells, which sometimes attain to several inches in length. Each node, or zone, from which the branches spring, consists of a single plate, or layer, of small cells, which are a continuation of the cortical layer of the internode ([Fig. 290], No. 3) as an outgrowth. Each cell is partially filled with chlorophyll granules, and it is these that are seen under the microscope taking the course shown by the arrows ([Fig. 290], No. 2). The rate of movement of the granules is accelerated by moderate warmth and retarded by cold. It is in viewing the circulation in water plants that the warm stage of the microscope is brought into use. Borne along with the protoplasmic stream are a number of solid particles consisting of starch granules and other matters. The method of viewing the circulation is by cutting sections off a portion of the plant with a very sharp knife, and arranging them in a growing cell with a few drops of water, and covering over with a thin cover-glass.
Fig. 291.—The Fructification of Chara fragilis.
A. Portion of filament containing “antheroids”; B. A group of antheridial filaments, composed of a series of cells, within each of which antherozoids are formed; C. The escape of mature antherozoids, with whip-like prolongations, about to swim off; D. Antherid supported on flask-shaped pedicle; E. Nucule enlarging, and seen to contain oospores; F. Spores and elaters of Equisetum; G. Spores surrounded by elaters of Equisetum.
The reproductive process of Chara is effected by two sets of bodies, both of which are placed at the base of the branches ([Fig. 291], E and D) either on the same or different plants, one set known as globules or antherids, and the other as nucules, containing the oospores or archegones. These are often of a bright red colour, and have covering plates, or shields (B and E), curiously marked, and the central portion is composed of a number of filaments rolled up (as in E) or free (as seen at B), projecting out from the centre of the sphere. The antherid is supported on a short flask-shaped pedicle, which projects into the interior. At the apex of each of the eight manubria is a roundish hyaline cell, termed a capitulum, and at its apex again six smaller or secondary capitula. The long whip-shaped filaments are divided by transverse septa into a hundred or more compartments, every one of which is filled with an antherozoid (as at A), consisting of a spiral thread of protoplasm packed into two or three coils; these escape and become free (as seen at C), each having two long fine flagella. The young antherozoid swims off with a lashing action, and the whole field appears for a time filled with life. They swim about freely, but their motion gradually ceases, and soon they arrive at a state of inaction.