234 PART III. THE CLASSIFICATION OF PLANTS. 



tissues of higher plants ; thus, Nostoc is constantly found in the 

 tissue of certain Hepaticae (Blasia and Anthoceros), in the porous 

 cells of the leaves of Sphagnum (Moss), and in the tissue of the 

 stem of Gunnera (Dicotyledon). 



The Cyanophyceae are both marine and fresh- water ; many grow- 

 on damp walls, rocks, etc. 



Sub-Class II. CHLOROPHYCEJ;, or Green Algae. In the simpler 

 forms the plant (that is, the gametophyte in all species which are 

 sexual) consists of a single cell (e.g. Protococcoideae, some Desmi- 

 dieae) ; or it is coenocytic, as in the Siphonoideae, either unseptate 

 (Siphoriaceae) or incompletely septate (Cladophoraceae, Hydrodic- 

 tyaceae) ; it is, in fact, only in this sub- class that the coenocytic 

 structure occurs among the Algse ; or the body is multicellular, 

 with essentially similar cells and therefore coenobitic (e.g. Spiro- 

 gyra, Pandorina* Ulva), or exhibiting at least a distinction be- 

 tween vegetative and reproductive cells (e.g. Volvox, Chara). The 

 only members of the sub-class in which there is any appreciable 

 differentiation of the vegetative cells are the Characeae. 



The body presents all degrees of morphological differentiation ; 

 it may be a thallus, either spherical (e.g. Haematococcus, Volvox), 

 or filamentous (e.g. Spirogyra, Ulothrix), or a flattened expansion 

 (e.g. Ulva, Coleochaete) ; or a filament with rudimentary differen- 

 tiation into root and shoot (e.g. (Edogonium) ; or it may present 

 differentiation into stem, leaf, and root (e.g. Caulerpa, Fig. 162, 

 Characeae). It may be free or attached. Growth and cell-division 

 commonly go on in all the cells of the body, so that the growth is 

 intercalary (e.g. Spirogyra, (Edogonium, Ulva); it is but rarely that 

 there is a definite growing-point, and then it is apical (Coleochaate, 

 Characeae, some Siphonoideae) ; and in the cellular plants which 

 have an apical growing-point, there is a single apical cell. 



The sporophyte is very simple, both histologically and morpho- 

 logically, in this sub-class. In most cases it is represented by the 

 sexually-produced spore (e.g. Pandorina, CEdogonium, Sphaeroplea) 

 the contents of which undergo division, and, on germination, are 

 set free as zoospores. In Coleochaete alone does the oospore under- 

 go division with the formation of septa, so that the sporophyte 

 is multicellular, but even in this case is quite rudimentary. 



Vegetative multiplication by division occurs in some of the lower 

 forms (e.g. Protococcoideae) of this sub-class. Reproduction by 

 zoospores and zoogonidia is general (absent in Pleurococcaceae, 

 Conjugatse, most Volvocoideae. Characeae); they are formed, not in 



