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 rounded 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 waU of the spore cracks 



Fig. 791. Filaments from a SoUoc colony. ^nd ceU-divisions take 

 After Luerssen. place in the cell in such 



order that a filament is 

 produced much resemblmg an ordinary hormogonium, which 

 becomes set free from the 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 the group. 



In habit, some of the Cyanophycese 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 symbioticaUy 

 with other plants. Thus Nostoc is frequently found associated 

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

 in the thaUus of Azolla and in the body 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 thaUus. 



Sub-Class II. — DiATOMACEffl. 



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 Algse, 

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



