THALLOPHYTA— ALG.E 39 



produced which are miilticeUular. Chara gives rise also to 

 pecnhar structures, the embryonic branches, which, are referable 

 to this mode of reproduction. 



The main sub-divisions of the Algae 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 

 histor}' which warrant their being considered as sub-classes. 

 The divisions are Cyanophyceae (blue green), Chlorophyceae 

 (green), Phseophycese, and Diatomacege (olive-brown), and 

 Khodophyceae (red). 



Sub-Class I. — Cyanophyce.e. 

 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 cells 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 cases 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 filaments 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- 

 cj'anin or blue-green colouring matter is diffused through the 

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

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

 while the sheath is alwaj^s 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 Jwrmogonia. 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 hla- 

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

 heterocysts. These mark the limits of the hormogonia. 



The filament sometimes branches, either regularly by the 



