GROUPS OF PLANTS. 41 



green Algae. This body is not strictly a spore but is in the nature 

 of a resting cell (Fig. 26a). 



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, water, ammonia, etc. They 



Fig. 26a. Bacillus sttbtiiis (hay bacillus), a, Small rod-like organisms such as are 

 found in an infusion of hay, or bouillon; b, zooglcea or mass of bacilli forming the " skin " 

 on the surface of infusions; c, chains of organisms forming spores; d, individual bacilli 

 showing flagella, which are only seen after staining. After Migula. 



serve a useful purpose in many technical operations, as in the 

 making of cheese, acetic acid, fermentation of tobacco, curing of 

 vanilla and many vegetable drugs, and in soil nitrification, helping 

 to change ammonia into nitrates- one of the sources of the nitro- 

 gen used by plants (see page 98). Many of them are disease- 

 producing, or pathogenic, and are the cause of a number of infec- 

 tious diseases in man and the lower animals, and plants as well. 

 They are injurious in two ways, in one case they consume the 



