12 A TEXT-BOOK OF BOTANY. 



size at one period, later they may be as large as peas or cherries. 

 Owing to their variation in appearance in different seasons various 

 names have been given to the same form by different investigators. 

 They are also associated with lichens. According to systematists, 

 the forms of Nostoc are arranged according to their aquatic or 

 terrestrial habits. 



SCHIZOMYCETES, OR BACTERIA. The Bacteria, or 

 Fission Fungi, occupy rather an anomalous position, some writers 

 classifying them with Fungi and some with Algae. They are i- 

 celled plants, microscopic in size, and of various shape. The con- 

 tents consist of protoplasm and a central body in some cases, which 

 is looked upon as a rudimentary nucleus. They are more or less 

 colorless, but sometimes produce a distinct pigment called bacterio- 

 purpurin which is rose-red or violet, and occasionally a chlorophyll- 

 green color substance. They are capable of multiplying by division 

 in one. two, or three directions, and under favorable conditions in- 

 crease very rapidly in number. The wall is more or less albumin- 

 ous in character, in this respect resembling the wall of the animal 

 cell, and is provided with one or more cilia, or flagella, the number 

 and position of which have been used as a basis of classification. 

 Sometimes the walls of the cells become mucilaginous, so that the 

 bacteria hold together, forming a mass known as a zoogloea. 

 Bacteria may form resting spores which arise in two ways. In 

 one case the contents round off and take on a membrane forming 

 a so-called ENDOSPORE ; in the other case the plant body is trans- 

 formed directly into a spore known as an ARTHROSPORE, as in 

 some of the Blue-green Algae. This body is not strictly a spore, 

 but is in the nature of a resting cell (Fig. 7). Two classes of 

 bacteria are frequently distinguished, as follows : Aerobic, or 

 those which require oxygen for their development and conse- 

 quently grow best when they have access to air or oxygen ; and 

 anaerobic, or those whose development is accelerated under re- 

 verse conditions, as in underlying tissues or in the interior of 

 cultures. 



Occurrence. Bacteria occur everywhere in nature, and play 

 a most important part in decay and putrefaction, in that they 

 change dead animal and plant tissues back again into simple inor- 

 ganic substances, as carbon dioxide,, hydrogen, water, ammonia, 



