14 MORPHOLOGY 



acid so that nitrites are formed, when these in turn are oxidized to ni- 

 trates, which are nitrogen-containing compounds available for green 

 plants. 



Iron bacteria. — These forms live in iron-containing waters, and as a result 

 of their activities iron oxid is deposited in the gelatinous matrix. This char- 

 acteristic reddish slimy deposit is exceedingly common about iron springs and 

 their outlets. 



Sulphur bacteria. — These bacteria are able to oxidize sulphuretted hydrogen, 

 storing free sulphur in their cells. Most conspicuous among them is a high-grade 

 filamentous form (Beggiatoa) closely resembling the filamentous Cyanophyceae. 



These statements illustrate the remarkable powers found among 

 bacteria, and when they are grouped together, the list is a striking one. 

 A group which is remarkably resistant to external conditions that 

 destroy other plants, which can manufacture carbohydrate food with- 

 out chlorophyll or light, which can use free nitrogen, which can live 

 without oxygen, is suggestive of the possibilities of plant life under 

 conditions that would forbid all existing vegetation. 



Myxobacteriaceae. — This group of organisms has been recognized recently, 

 and is evidently related to the bacteria, as the name suggests ; but it is dis- 

 tinguished from them by a remarkably complex colony organization. The indi- 

 vidual cells resemble those of the bacteria, but they are combined in structures of 

 definite and often elaborate form. For example, one colony resembles a stalk 

 bearing a group of sporangia at its summit. The life histories of the individual 

 cells are like those of the bacteria, and resting cells (so-called spores) are formed 

 in the same way, from which rodlike cells escape and assemble to organize the 

 complex colony. The name suggests a combination of the characters of slime 

 molds and bacteria, the individual cells resembling the latter, and the cells coming 

 together to form a complex body, as the plasmodium of the slime molds is formed. 

 The group is included here because of its resemblance to the bacteria, but it must 

 not be inferred that it belongs to the schizomycetes or even to the schizophytes. 

 It must remain at present as one of the thallophyte groups of uncertain position. 



3. ALGAE 



These make up the great chlorophyll-bearing assemblage of thallo- 

 phytes, capable of manufacturing food (see p. 363), and representing the 

 forms from which the higher groups of plants have probably been 

 derived. The three groups of algae are named from their characteristic 

 pigments, as follows: Chlorophyceae (green algae), Phaeophyceae 

 (brown algae), and Rhodophyceae (red algae). These differences in 

 pigments are associated with important differences in structure, which 

 will appear as the forms are discussed. 



