The Plant Kingdom - 595 



MATRIX _ 

 SHEATHS- 

 CELL 





OSCILLATORIA ^y\. , 



A 



MATRIX 

 SHEATH 



GLEOCAPSA 



NOSTOC 



Fig. 31-1. Some blue-green algae (Cyanophyta). Note the absence of nuclei and chloroplasts in the primitive 

 cells. None of the blue-greens reproduces sexually. 



the most ancient and primitive of all green 

 plants. Most of the 2000 or so known species 

 display a bluish-green color, owing to the 

 presence of a blue pigment, phycocyanin, 

 which is mixed with the chlorophyll. How- 

 ever, red and yellow pigments are also found 

 in the cytoplasm, and some of the species 

 are reddish and a few are brown-green. Many 

 cyanophytes live in fresh waters. In reser- 

 voirs and lakes, they sometimes pollute the 

 water seriously by their great abundance. 

 Also there is a large number of marine spe- 

 cies. Occasionally a red species, prevalent in 

 the Red Sea, flourishes in such numbers that 

 the water actually becomes colored. A few 

 blue-greens live on damp soil, in moist shady 

 spots; or on the damp outer surfaces of 

 flowerpots in humid greenhouses. 



The primitive status of the blue-green 

 algae is indicated by the fact that none of 

 them is multicellular. Many are of solitary, 

 unicellular habit, but others form colonial 

 aggregates, such as are shown in Figure 31-1. 

 Frequently the cells of the colony are pro- 

 tected by one or more gelatinous coverings 

 of pectinous material. The colonies may be 

 threadlike (filamentous), or they may be 

 more or less ball-like (Fig. 31-1). 



Another primitive feature of the cyano- 

 phytes is their extremely simple cell struc- 



ture. Definite nuclei have not been devel- 

 oped in the cells. The granules of nuclear 

 material (chromidia) are not enclosed 

 within a nuclear membrane. They tend to be 

 scattered more or less evenly throughout the 

 cytoplasm (Fig. 31-1). However, in some spe- 

 cies (Fig. 2-3), the chromidial granules may 

 be aggregated toward the center of the cell — 

 a sort of transitional stage in the evolution 

 of true nuclei. Moreover, the cells lack chlo- 

 roplasts. The chlorophyll and other pigments 

 appear to be dissolved, or at least very finely 

 distributed, throughout all parts of the 

 cytoplasm. 



Further evidence as to the primitive status 

 of the cyanophytes is that sexual reproduc- 

 tion has never been found in any of the 

 species. Apparently this group of plants 

 branched off and took an independent line 

 of evolution at a very early date — before 

 sexual reproduction had been evolved by the 

 ancient ancestral organisms (see Fig. 29-11). 



The great antiquity of the Cyanophyta is 

 further shown by the paleontological record. 

 Some representatives of the blue-green algae 

 are found in the very oldest of fossil-bearing 

 strata, formed during the Proterozoic era 

 (p. 565). Among all the many fossils formed 

 by green plants in past ages, those of the 

 cyanophytes are by far the oldest. 



