DEVELOPMENT OF PLANTS 165 



CLASS B. BLUE GREEN ALGAE OR CYANOPHYCEAE 



63. The Structure and Nature of the Cyanophyceae. These 

 plants are very simple organisms that have many features in 

 common with the bacteria. They are unicellular plants, al- 

 though the cells are more commonly joined into rows, forming a 

 thread or filament (Fig. 92, A, B). In some cases these filaments 

 may be regarded as multicellular plants since they branch and 

 because the various cells perform different functions. They form 

 slimy, blue-green, black, yellow or violet masses and, like the 

 bacteria, appear to be dependent to a degree on organic material. 

 At least they are especially abundant in the presence of decaying 

 organic matter, as in the drainage from stables and watering 

 troughs, or in pools, puddles and damp places where there is an 

 abundance of filth and decay. The enormous increase of these 

 plants often produces a discoloration of the water, as in the Red 

 Sea, and they are often the cause of the foul odors of muddy 

 ponds and streams and reservoirs and of the pollution commonly 

 known as water-bloom or working of ponds. Like the bacteria 

 also they are associated with varying amounts of gelatinous sub- 

 stances derived from their walls or excreted from the cells and in 

 both cases the walls may become colored with various pigments. 

 Doubtless the mucilaginous character of these plants enables 

 them to retain moisture and so adapts them to dryer conditions 

 than would otherwise be possible. The Cyanophyceae differ 

 radically from the bacteria in possessing chlorophyll which is dis- 

 tributed in the outer portion of the protoplasm, and in the pos- 

 session of a more definite nucleus. A blue pigment, phycocyanine, 

 is associated with the chlorophyll and for this reason they are 

 called the blue-green algae or Cyanophyceae. Cell division fol- 

 lows the ordinary method noted in the higher plants, at least this 

 appears to be true in those cases that have been accurately 

 studied. These plants accordingly show a decided advance over 

 the preceding forms not only in the differentiation of the cells 

 but especially because they are capable, to a degree at least, of 

 manufacturing food from inorganic substances by reason of their 

 chlorophyll. 



In addition to the multiplication of individuals by cell divi- 



