174 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



mentous species are usually enveloped by a gelatinous sheath that may be homogeneous 

 or stratified and frequently is pigmented. The inner portion of the sheath contains a 

 small amount of cellulose (Kylin, 1943a). 



In the majority of bluegreen algae, two regions are distinguishable within the proto- 

 plast: a peripheral region, the chromoplasm, which contains the pigments, and a colorless 

 central region, the central body or centroplasm. Ordinarily the protoplast contains no 

 evident vacuoles. 



Recent work indicates that the pigments in the chromoplasm occur in small bodies 

 (Calvin and Lynch, 1952). The pigments consist as far as known of chlorophyll a, beta- 

 carotene and another carotene (flavacin) found only in Schizophyceae, two xanthophylls 

 that are peculiar to these algae, and two proteinaceous pigments (phycobilins), the blue 

 c-phycocyanin and the red c-phycoerythrin (Strain, 1951, p. 253). 



The centroplasm contains various kinds of bodies, including some that are in the form 

 of granules, rods, or reticula and become evident after application of the Feulgen nuclear 

 reaction. The bodies are not bounded by a nuclear membrane, however, and no nucleoli 

 appear to be present. 



In the Stigonemataceae and certain Scytonemataceae adjoining cells are united by 

 pit connections. Only one such connection is present between any two cells and it is 

 always a primary pit connection. 



All of the filamentous bluegreen algae, with the exception of the Oscillatoriaceae, 

 regularly form a special type of cell known as a heterocyst. They originate from vege- 

 tative cells and have a thickened wall. Intercalary heterocysts have a conspicuous pore 

 at each end; terminal heterocysts have a pore only at the proximal end. The filaments 

 frequently break at the heterocysts and these structures indirectly function in vegetative 

 multiplication. In some instances they have been observed to produce new filaments. (See 

 Fritsch, 1951, for a discussion on these cells.) 



As far as known sexual reproduction does not occur in the Schizophyceae. Vegetative 

 multiplication by fission in the unicellular or colonial forms is of common occurrence. 

 In the order Hormogonales the chief method of multiplication is by short lengths of the 

 vegetative filaments called hormogonia. The hormogonia are delimited by the death of 

 occasional cells at intervals along the length of the filament. In certain forms the hormo- 

 gonia are modified as organs of perrenation (hormocysts). 



Many of the filamentous genera, but no Oscillatoriaceae, form thick-walled resting 

 spores known as akinetes. In certain genera of the Chamaesiphonales the contents of a 

 cell divide into a number of endospores. These spores are thin-walled and germinate 

 directly to produce new plants. 



The bluegreen algae live in many kinds of habitats. Many are aquatic in freshwater 

 or marine situations; others are terrestrial or subaerial in occurrence. A number of forms 

 grow in hot springs, at times with a temperature as high as 85° C. Many forms live in 

 association with other organisms — plants and animals. Species belonging principally 

 to the genera Gloeocapsa, Nostoc, Scytonema, and Stigonevia constitute the algal associate 

 in many lichens. A number of species are able to fix atmospheric nitrogen. 



History: Although members of this class have been known to the world of 

 science since the time of Linnaeus (1753), who described a few species under 

 the generic names Tremella, Byssus, and Viva, the distinctive features of the 

 class remained unrecognized until the middle of the nineteenth century. To be 

 true C. Agardh in 1824 established an order Nostochinae (one of six into which 

 he divided the algae) to receive Nostoc and Bivularia, but he also assigned to 

 this order several genera belonging to unrelated groups of algae and referred 

 other genera of bluegreen algae (e.g., OsciUatoria) to the order Confervoideae. 



The bluegreen algae were first recognized as constituting an autonomous 

 group (order) of algae by Stizenberger in 1860 (p. 18). He called them Myxo- 

 phyceae, adopting a designation (Myxophykea) previously used by Wallroth 

 (1833, p. ix) for a heterogeneous assemblage of algae, including representatives 



