14 ] The Classification of Lower Organisms 



bell shaped bodies. He named these granules mychits. It might be held that the 

 mychit is a chromosome, and the central body of bacteria a nucleus of a single 

 chromosome, if it were not true that the blue-green algae contain comparable bodies 

 of variable form and indefinite number. 



Many bacteria swim by means of flagella. The diameter of the flagella, as revealed 

 by the electron microscope, is of the order of 0.02 [J.. Their positions and lengths were 

 made known, before the invention of the electron microscope, by the technique of 

 Loeffler (1889), which consists essentially of depositing upon them a heavy layer 

 of tannic acid. By the absence or presence and arrangement of flagella, bacteria are 

 classified as of four types: atrichous, without flagella; monotrichous, with one flagel- 

 lum at one end; lophotrichous, with a tuft of flagella at one end; peritrichous, with 

 flagella on the sides. 



Myxobacteria, spirochaets, and such blue-green algae as are sheathless filaments, 

 are capable of bending movements (some spirochaets, observed with the electron 

 microscope, are found also to have flagella at the ends of the cells). Spirochaets swim 

 vigorously; in myxobacteria and blue-green algae, the bending movements are a mat- 

 ter of slow writhing. Filaments and cells of blue-green algae are capable also of a 

 moderately rapid gliding movement. The mechanism of this movement has been 

 the subject of much speculation, reviewed by Burkholder ( 1934), but remains uncer- 

 tain. The appearance of the movement is as though it were caused by local secretion 

 of substances affecting surface tension. 



The normal reproduction of Mychota is by constriction of the cells, each into two 

 equal daughter cells; whence the various names in schizo- (Greek axi^co, to split). 

 Henrici (1928) studied the changes undergone by bacteria during multiplication. As 

 the cells become numerous, decreasing the food supply and producing substances 

 harmful to themselves, they begin to attain greater length before dividing. Subse- 

 quently there is a gradual transition to enlarged and distorted forms called involution 

 forms, which divide irregularly, cutting off minute fragments. These observations 

 suggest the idea that the involution forms are the true normal forms of bacteria, the 

 so-called normal forms being a temporary stage adapted to rapid multiplication 

 under favorable conditions. 



In many rod-shaped bacteria, when conditions cease to be ideal, the protoplasts 

 produce within themselves walled bodies of dehydrated protoplasm called spores 

 (endospores). In general, each cell produces only one spore. No experiment has 

 definitely shown how long these spores can remain alive; it is surely a matter of cen- 

 turies, doubtfully of millenia. 



Lohnis and Smith (1916, 1923) observed of Azotobactcr that numbers of proto- 

 plasts might escape from their walls and unite in a common mass, which they named 

 the symplasm. The existence of this stage has never been confirmed by other authori- 

 ties. If the symplasm exists, it is a device for achieving the effect which nucleate or- 

 ganisms attain by sexual reproduction, that is, combination of the heredity of differ- 

 ent lines of ancestry. 



Tliat Mychota can actually combine characters from different linos of ancestry 

 was first demonstrated beyond question by Tatum and Ledcrberg (1947). They 

 mixed cultures of pairs of varieties of Escherichia coli, differing in two or more 

 physiological characters, and isolated from the mixtures races having characters de- 

 rived from both components. Further Mork, reviewed by Ledcrberg and Tatum 

 (1953), has abundantly demonstrated phenomena analogous to typical sexual 

 reproduction. 



