DEVELOPMENT OF PLANTS 155 



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 substances de- 

 rived 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 dififer 

 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- 

 sion, as in the case of the bacteria, many of the thread-like forms 

 have another interesting method of increasing their numbers. 

 A few of the cells, termed a hormogonium, become separated 

 from the other cells and frequently possess for a time a slight 

 motility that enables them to move away from the parent fila- 

 ment (Fig. 92, hr). In some species one or several rather large, 

 colorless cells, heterocysts, appear in the filament and may serve 

 to separate it into hormogonia (Fig. 92, h). In Oscillatoria 

 (Fig. 93) the filaments into which these hormogonia grow per- 



