40 MANUAL OF BOTANY 



formation of growing points placed laterally, or irregularly often 



by the hormogonia not becoming completely detached, and so 



growing while adhering to the filament. 



In many forms spores are produced. These are single cells 



of the filament or mass, which are roimded in shape, larger than 



the other cells, and fur- 

 FiG. 791. nished with a strong 



thickened cell- wall. They 

 become detached from the 

 plant, and are capable of 

 resting for some time be- 

 fore germinating. In the 

 filamentous forms, when 

 germination takes place, 

 the wall of the spore cracks 



„. _„, r^■^ ^ t V , 1^ and cell- divisions take 



Fkj. 791. Filaments from a Avstoc colony. 



After Luerssen. place in the cell in such 



order that a filament is 

 produced much resembling an ordinary hormogonium, which 

 becomes set free from tlie spore-coating and develops into the 

 plant. In the other forms, the divisions of the germinating spore 

 are irregular and give rise to a mass of cells. Sexual reproduc- 

 tion is unknown in tlie group. 



In habit, some of the Cj^anophyceae are free floating or- 

 ganisms ; others are attached by their bases to rocks or stones. 

 Some are embedded by their gelatinous coatings to form colonies 

 of various shapes. Some are endophytic and live symbiotically 

 with other plants. Thus Nostoc is frequently found associated 

 with a fungus in the thallus of a lichen ; it occurs, too, in cavities 

 in the thallus of Azolla and in the bod}^ of Anthoceros. Others 

 again are epiphytic, bormg into the tissue of other Algae and 

 remaining attached to them. A few of them, chiefly abundant in 

 hot springs, are covered with a precipitate or pellicle of carbonate 

 of calcium, which may wrap round the individual filaments, or 

 enclose the whole thallus. 



Sub-Class II. — DlATOMACE^. 



These plants have often been included in the next sub-class, 

 the Phseophyceae, on account of their olive-brown colour. They 

 are, however, so unlike them in every other respect, while they 

 present no very great resemblance to any other group of Algae, 

 that it seems best to regard them as a separate sub-class. 



