=340. Family Hydrodictyaceæ.=—These plants form colonies of cells. Hydrodictyon reticulatum, the water net, is made up of large numbers of cylindrical cells so joined at their ends as to form a large open mesh or net. Pediastrum forms circular flat colonies, as shown in fig. 166. Both of these plants are rather common in fresh-water pools, the latter one intermingled with filamentous algæ, while the former forms large sheets or nets. Multiplication in Hydrodictyon takes place by the protoplasm in one of the cells dividing into thousands of minute cells, which gradually arrange themselves in the form of a net, escape together from the mother cell, and grow into a large net. In Pediastrum multiplication takes place in a similar way, but the protoplasm in each cell usually divides into sixteen small cells, and escaping together from the mother cell arrange themselves and grow to full size (fig. 166). =341. The Conjugateæ= include several families of green algæ, which probably should be included among the Chlorophyceæ. They have probably had their origin from some of the more simple members of the Protococcoideæ. They are represented by Spirogyra, Zygnema, and the desmids, studied in Chapter 14. =342. Subclass CONFERVOIDEÆ.=—These are mostly filamentous algæ, the filaments being composed of cells firmly united, and, with the exception of the simplest forms, there is a definite growing point. A few of the families are as follows: =343. Family Ulvaceæ.=—These contain the sea wracks, or sea lettuce, like Ulva, forming expanded green, ribbon-like growths in the sea. [Illustration: Fig. 167. Ulothrix zonata. _A_, base of thread. _B_, cells with zoospores, _C_, one cell with zoospores escaping another cell with small biciliate gametes escaping and some fusing to form zygospores, _E_, zoospores germinating and forming threads: _F_, _G_, zygospore growing and forming zoospores. (After Caldwell and Dodel-Port.)] =344. Family Ulotrichaceæ=, represented by Ulothrix zonata, not uncommon in slow-running water or in ponds of fresh water attached to rocks or wood. It consists of simple threads of short cells. Multiplication takes place by zoospores. Reproduction takes place by motile sexual cells (gametes) which fuse to form a zygospore (fig. 167). =345. Family Chætophoraceæ=, represented by Chætophora (in Chapter 15) and Drapernaudia in fresh water. =346. Family Œdogoniaceæ=, represented by Œdogonium (Chapter 16). =347. Family Coleochætaceæ=, represented by Coleochæte (Chapter 17). =348. Subclass SIPHONEÆ.=—There are several families. =349. Family Botrydiaceæ.=—This is represented by Botrydium granulatum (Chapter 15, p. 146). =350. Family Vaucheriaceæ=, represented by Vaucheria (Chapter 15), with quite a large number of species, is widely distributed. Class Schizophyceæ (= Cyanophyceæ). [Illustration: Fig. 168. Glœocapsa.]