THE ALGAE 105 



In Cladophora glomerata it has been shown recently that zoospore formation 

 occurs at intervals all through the year, while gamete formation happens only 

 in the spring and at the end of a long series of zoospore discharges. Gametes 

 and zoospores are developed in distinct plants, but there is no reduction 

 division in the formation of the zoospores. It appears that this species can 

 produce a succession of diploid zoospore generations, after which meiosis 

 occurs during the formation of the gametes. Conjugation restores the diploid 

 condition. This condition resembles that which has been described in some 

 members of the Siphonales. 



CYANOPHYCEAE (Myxophyceae) 



The Cyanophyceae are Algae in which the thallus may be either unicellular 

 or filamentous. It is always enclosed in a gelatinous sheath. The 

 cells possess phycocyanin (blue) and carotin (orange) in addition to chloro- 

 phyll, and the varying proportions of these pigments not only mask the 

 green colour but cause a wide variation of tint from almost pure blue to 

 brown. They occur chiefly in fresh water and on damp surfaces ; some are 

 found in the soil, whilst a minority are marine. They constitute the 

 principal vegetation of hot springs, some being able to live in water as high 

 as 85° C. 



Apart from their interest as the simplest of the Algae they are remarkable 

 for their cytological structure. The cell wall is usually very thin and is 

 composed either of pure cellulose or a hemi-cellulose. External to the wall 

 is the gelatinous sheath, composed of pectic compounds.* It is sometimes 

 thin and tough, sometimes massive and diffuse. It may also consist 

 of multiple lamellae, and not infrequently forms a common investment 

 round a group of filaments, or trichomes as they are called in this family. 

 These sheaths may become very tough, and it is undoubtedly due to 

 them that the plants can withstand high temperatures and considerable 

 desiccation. 



The protoplast is remarkably simple. It is more or less uniform, without 

 any vacuole or plastids. The central region is almost colourless, and scattered 

 in it are chromatin granules which represent all there is of a nucleus. These 

 granules can divide, and they separate into two groups prior to cell division, 

 but in some instances there appears to be little, if any, co-ordination between 

 the grouping of these granules and the division of the cells. There is certainly 

 no regular mitosis. In the external zone of the protoplasm the pigments are 

 diffused throughout its substance and there are also present numerous minute 

 granules, including oil drops, glycogen and others which are probably volutin 

 and are called metachromatic granules. 



Many of the filamentous species develop either terminal or intercalary 

 heterocysts, which are easily recognized by their larger size, thick wall and 

 homogeneous contents. The function of these bodies is not known, 



* A symbiosis with Nitrogen-fixing Bacteria, living on this sheath appears to be common 

 among the Cyanophyceae. 



