8 PLANT MORPHOLOGY 



ceae, Bacillariophyceae, Chlorophyceae, Charophyceae, Phaeophyceae, 



and Rhodophyceae. 



1. CYANOPHYCEAE 



The Cyanophyceae,^ or blue-green algae, are the simplest and lowest 

 group of green plants. They are characterized by having, in addition 

 to chlorophyll and carotinoids, a blue pigment, phycocyanin, the combina- 

 tion resulting in a blue-green color. Some of the Cyanophyceae, how- 

 ever, also possess a red pigment, phycoerythrin, the presence of which, in 

 varying amounts, produces shades of red, brown, or purple. The Red 

 Sea is said to have received its name from a floating species, Trichodes- 

 mium erythraeum, which is red and sometimes occurs in such abundance 

 as to color the water. The Cyanophyceae are unicellular plants, the 

 cells being nearly always grouped to form colonies of various kinds. 

 About 1,500 species are known. 



The Cyanophyceae comprise two orders: (1) the Coccogonales, whose 

 cells are either solitary or arranged in nonfilamentous colonies; and (2) 

 the Hormogonales, whose cells are in filamentous colonies. Some of the 

 genera belonging to the Coccogonales are Chroococcus, Gloeocapsa, Meris- 

 mopedia, Coelosphaerium, and Chamae siphon. The main genera of the 

 Hormogonales include Oscillatoria, Lynghya, Nostoc, Anahaena, Rivula- 

 ria, Gloeotrichia, Tolypothrix, Scytonema, and Stigonema. 



Distribution and Habitat. Blue-green algae are found in all parts of 

 the world where plants can grow. Most of them live in fresh water, 

 some occur on moist earth, rocks, and trees, while others live in the ocean. 

 They commonly form scums, slimy mats, or gelatinous lumps. They 

 are especially prevalent in stagnant water, where large quantities of 

 organic matter accumulate. Some live in hot springs at temperatures 

 as high as 75°C. Many forms extract calcium and magnesium from the 

 water and cause minerals, which are often brightly colored, to be depos- 

 ited on rocks in the vicinity. Some species of Nostoc and Anahaena live 

 as endophytes in the intercellular cavities of other plants, as in the 

 thallus of Anthoceros, the leaves of Azolla, and the roots of cycads. Some 

 blue-green algae enter into the formation of lichens. 



The Cyanophyceae living in hot springs thrive under conditions that 



1 Sometimes called Schizophyceae or Myxophyceae. Schizophyceae means "split- 

 ting algae"; Myxophyceae means "slime algae." These names are used by some 

 botanists in preference to Cyanophyceae, which means "blue algae," because not all 

 the members are blue-green. But it is also true that some of them lack the slippery 

 feel. As long as we retain the names Chlorophyceae, Phaeophyceae, and Rhodo- 

 phyceae for other algal groups, we might as well retain the name Cyanophyceae for 

 the sake of uniformity, especially since some of the Chlorophyceae are not green, 

 some of the Phaeophyceae are not brown, and some of the Rhodophyceae are not red. 

 The things most desired in a name are that it shall express the most prominent feature 

 of the group and that it shall be consistent with the names of coordinate groups. 



