THALLOPHYTA— ALGiE 39 



produced whioli are multicellular. Chara gives rise also to 

 peculiar structures, the embryonic branches, which are referable 

 to this mode of reproduction. 



The main sub-divisions of the Algse are based upon the colour 

 of the thallus. This distinction would be of little value in itself, 

 but the groups so defined show peculiarities of form and of life 

 history which warrant their being considered as sub-classes. 

 The divisions are Cyanophycea; (blue green), , Chlorophyceae 

 (green), Phseophycefe, and Diatomaoes (olive-brown), and 

 Ehodophycese (red). 



Sub-Class I. — Cyanophycea;. 



The forms presented by the members of this group are very 

 simple. In many cases the plant is unicellular, or if it divides, 

 the products of the division remain surrounded by a common 

 cell-wall, which becomes mucilaginous and of considerable thick- 

 ness, so that the oeJls appear as if embedded in a mass of jelly. 

 The plane of division may be such as to cause the cells to 

 form a flat plate, or a mass more than one cell in thickness 

 may result. In other oases the cells divide so as to form a 

 filament, often of some length, sometimes tapering at its apex, 

 which may be either free or attached at its base to some substra- 

 tum. Many filaments usually are collected together, so that 

 the plant appears in tufts. This filamentous form is also invested 

 with a sheath, of similar consistence to that in the former case. 

 The sheath is often coloured with various tints, purple, blueish, 

 or red. The cells are not ciliated, but the filanjents are capable 

 of a peculiar swaying movement, the mechanism of which is not 

 known. The cells are in all cases of very simple structure ; no 

 nucleus has been satisfactorily shown to exist, and the phyco- 

 cyanin or blue-green colouring matter is difl'used through the 

 protoplasm ; no chromoplastids have been found. The cell- wall 

 has been stated to be formed of a, substance resembhng cutin, 

 while the sheath is always cellulose, or a modification of it. The 

 reproductive processes are chiefly vegetative ; the filaments split 

 up into fragments, which, after a period of inactivity, grow out 

 into filaments like their parent. These fragments, which consist 

 of several cells, are called hormogonia. In one section of the 

 sub-class, to which Nostoc belongs, the cells of the filament are 

 of two kinds {fig. 791) ; here and there in the course of the fila- 

 ment large, almost spherical clear cells appear, which are named 

 heterocysts. These mark the limits of the hormogonia. 



The filament sometimes branches, either regularly by the, 



