LITMIR.BIZ

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some species the sexual organs are borne directly on the filament (Fig. 21, G).

      Large zoöspores are formed in some of the green felts (Fig. 22, A), and are produced singly in the ends of branches that become swollen, dark green, and filled with very dense protoplasm. This end becomes separated by a wall from the rest of the branch, the end opens, and the contents escape as a very large zoöspore, covered with numerous short cilia (A ii). After a short period of activity, this loses its cilia, develops a wall, and begins to grow (III, IV). Other species (B) produce similar spores, which, however, are not motile, and remain within the mother cell until they are set free by the decay of its wall.

      Order V.—Characeæ.

      The Characeæ, or stone-worts, as some of them are called, are so very different from the other green algæ that it is highly probable that they should be separated from them.

      The type of the order is the genus Chara (Fig. 23), called stone-worts from the coating of carbonate of lime found in most of them, giving them a harsh, stony texture. Several species are common growing upon the bottom of ponds and slow streams, and range in size from a few centimetres to a metre or more in height.

      The plant (Fig. 23, A) consists of a central jointed axis with circles of leaves at each joint or node. The distance between the nodes (internodes) may in the larger species reach a length of several centimetres. The leaves are slender, cylindrical structures, and like the stem divided into nodes and internodes, and have at the nodes delicate leaflets.

      At each joint of the leaf, in fruiting specimens, attached to the inner side, are borne two small, roundish bodies, in the commoner species of a reddish color (Fig. 23, A, r). The lower of the two is globular, and bright scarlet in color; the other, more oval and duller.

      Examined with a lens the main axis presents a striated appearance. The whole plant is harsh to the touch and brittle, owing to the limy coating. It is fastened to the ground by fine, colorless hairs, or rootlets.

      Fig. 23.—A, plant of a stone-wort (Chara), one-half natural size. r, reproductive organs. B, longitudinal section through the apex. S, apical cell. x, nodes. y, internodes. C, a young leaf. D, cross section of an internode. E, of a node of a somewhat older leaf. F, G, young sexual organs seen in optical section. o, oögonium. An. antheridium. H, superficial view. G, I, group of filaments containing spermatozoids. J, a small portion of one of these more magnified, showing a spermatozoid in each cell. K, free spermatozoids. L, a piece of a leaf with ripe oögonium (o), and antheridium (An.). B, H, × 150. J, K, × 300. I, × 50. L, × 25.

      By making a series of longitudinal sections with a sharp razor through the top of the plant, and magnifying sufficiently, it is found to end in a single, nearly hemispherical cell (Fig. 23, B, S). This from its position is called the “apical cell,” and from it are derived all the tissues of the plant. Segments are cut off from its base, and these divide again into two by a wall parallel to the first. Of the two cells thus formed one undergoes no further division and forms the central cell of an internode (y); the other divides repeatedly, forming a node or joint (x).

      As the arrangement of these cells is essentially the same in the leaves and stem, we will examine it in the former, as by cutting several cross-sections of the whole bunch of young leaves near the top of the plant, we shall pretty certainly get some sections through a joint. The arrangement is shown in Figure 23, E.

      As the stem grows, a covering is formed over the large internodal cell (y) by the growth of cells from the nodes. These grow both from above and below, meeting in the middle of the internode and completely hiding the long axial cell. A section across the internode shows the large axial cell (y) surrounded by the regularly arranged cells of the covering or cortex (Fig. 23, D).

      All the cells contain a layer of protoplasm next the wall with numerous oval chloroplasts. If the cells are uninjured, they often show a very marked movement of the protoplasm. These movements are best seen, however, in forms like Nitella, where the long internodal cells are not covered with a cortex. In Chara they are most evident in the root hairs that fasten the plant to the ground.

      The growth of the leaves is almost identical with that of the stem, but the apical growth is limited, and the apical cell becomes finally very long and pointed (Fig. 23, C). In some species the chloroplasts are reddish in the young cells, assuming their green color as the cells approach maturity.

      The plant multiplies non-sexually by means of special branches that may become detached, but there are no non-sexual spores formed.

      The sexual organs have already been noticed arising in pairs at the joints of the leaves. The oögonium is formed above, the antheridium below.

      The young oögonium (F, O) consists of a central cell, below which is a smaller one surrounded by a circle of five others, which do not at first project above the central cell, but later completely envelop it (G). Each of these five cells early becomes divided into an upper and a lower one, the latter becoming twisted as it elongates, and the central cell later has a small cell cut off from its base by an oblique wall. The central cell forms the egg cell, which in the ripe oögonium (L, O) is surrounded by five, spirally twisted cells, and crowned by a circle of five smaller ones, which become of a yellowish color when full grown. They separate at the time of fertilization to allow the spermatozoids to enter the oögonium.

      The antheridium consists at first of a basal cell and a terminal one. The latter, which is nearly globular, divides into eight nearly similar cells by walls passing through the centre. In each of these eight cells two walls are next formed parallel to the outer surface, so that the antheridium (apart from the basal cell) contains twenty-four cells arranged in three concentric series (G, an.). These cells, especially the outer ones, develop a great amount of a red pigment, giving the antheridium its characteristic color.

      The diameter of the antheridium now increases rapidly, and the central cells separate, leaving a large space within. Of the inner cells, the second series, while not increasing in diameter, elongate, assuming an oblong form, and from the innermost are developed long filaments (I, J) composed of a single row of cells, in each of which is formed a spermatozoid.

      The eight outer cells are nearly triangular in outline, fitting together by deeply indented margins, and having the oblong cells with the attached filaments upon their inner faces.

      If a ripe antheridium is crushed in a drop of water, after lying a few minutes the spermatozoids will escape through small openings in the side of the cells. They are much larger than any we have met with. Each is a colorless, spiral thread with about three coils, one end being somewhat dilated with a few granules; the other more pointed, and bearing two extremely long and delicate cilia (K). To see the cilia it is necessary to kill the spermatozoids with iodine or some other reagent.

      After fertilization the outer cells of the oögonium become very hard, and the whole falls off, germinating after a sufficient period of rest.

      According to the accounts of Pringsheim and others, the young

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