Order 1. =Characeæ.= Algæ with a peculiar odour, often incrusted with lime, and of a brittle nature. They generally grow gregariously in large masses at the bottom of fresh and brackish water, and are from a few inches to more than a foot in height. The stem has long internodes which in _Nitella_ are formed of one cylindrical cell; in _Chara_ of a similar cell, but closely surrounded by a cortical layer of smaller ones. The protoplasm in contact with the cell-wall exhibits in a well-marked degree the movement of rotation (cyclosis), carrying the chlorophyll corpuscles along with it. The internodes are separated from each other by a layer of small cells (nodal cells) from which the leaves are produced. The leaves are borne in whorls of from 5–12 which regularly alternate with one another as in the higher verticillate plants; a branch is borne in the axil of the first formed leaf of each whorl (Fig. 61 _A_, _n_). [Illustration: FIG. 61.--_Chara fragilis. A_ Portion of a plant, natural size. _B_ Portion of a leaf _b_, with leaflets β′-β′′; _a_ antheridium; _c_ oogonium. _C_ A shield.--_Nitella flexilis. D_ Filament from antheridium with spermatozoids. _E_ Free spermatozoids.] The leaves are constructed in the same manner as the stem; they are divided into a series of joints, but have only a limited power of growth; their terminal cell, too, is not enclosed by a cortex. Leaflets are borne at their nodes. The growth of the stem is unlimited, and proceeds by means of an apical cell (Fig. 62 _s_). The apical cell divides into a segment-cell and a new apical cell. The segment-cell then divides by a transverse wall into two cells, one lying above the other; the lower one, without any further division, becomes one of the long, cylindrical, internodal cells (Fig. 62 _in_), and the upper one (Fig. 62 _n_) divides by vertical walls to form the nodal cells. The cortical cells (Fig. 62 _r_) which surround the long internodal cells of _Chara_, are derived from the divisions of the nodal cells; the cells covering the upper portion of an internodal cell being derived from the node immediately above it, and those in the lower part of the internode from the node below it. [Illustration: FIG. 62.--_Chara fragilis_: _s_ apical cell; _n_, _n_ nodal cells; _in_ internodal cells; _bl_, _bl_ leaves; _r_, _r_ the cortical cells.] [Illustration: FIG. 63.--Oogonium of _Chara_: _k_ “crown”; _u_ receptive spot; _s_ spermatozoids.]