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. [Illustration: 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.]