602 - Heredity and Evolution 



lack chlorophyll. Previously it was thought 

 that bacteria do not possess definitive nuclei, 

 but true nuclei have now been demonstrated 

 in many forms. Sexual reproduction may 

 occur, however, at least in primitive form. 



In the absence of chlorophyll, bacterial 

 nutrition is saprophytic (p. 174), parasitic (p. 

 178), or chemotrophic (p. 178). The dominant 

 forms, of course, are spherical (coccus), rod- 

 shaped (bacillus), and spiral (spirillum), as 

 is shown in Figure 10-1. Many bacteria are 

 solitary and unicellular, but many others form 

 colonial aggregates. Often the form of such 

 colonies may be inferred from the name; 

 for example, Streptococcus or Streptobacillus 

 indicates chainlike formations, and Staphylo- 

 coccus indicates an irregularly bunched col- 

 ony. Many species are flagellated. This is 

 particularly true of the spirillum forms, 

 which seldom, if ever, form colonial aggre- 

 gates. Bacterial cells are very small. The av- 

 erage size is about 1 to 3 microns in diameter 

 or length. 



As was explained in Chapter 10, the sapro- 

 phytic bacteria are very important because 

 they are responsible (mainly) for the proc- 

 esses of decay, putrefaction, and fermenta- 

 tion. Vast quantities of organic materials, 

 which otherwise would tend to accumulate 

 in the environment, thus undergo decom- 

 position, liberating their constituents in in- 

 organic form, which can be reutilized by 

 various other organisms (pp. 183-185). Also 

 the saprophytic bacteria are important in 

 relation to food spoilage (p, 178). And last 

 but not least, the parasitic bacteria are very 

 important as disease-causing agents in many 

 plants and animals, including man (Table 

 30-1). 



Some biologists include the Rickettsia 

 among the Schizomycophyta, but the classi- 

 fication of these intracellular parasites is 

 problematical. The Rickettsia were named 

 after their discoverer, Howard Ricketts, who 

 died in 1910 while studying typhus, which 

 is caused by one of these "organisms." Gen- 

 erally speaking, the Rickettsia, like viruses, 

 seem incapable of multiplying except when 



they are in the protoplasm of some living 

 cell. Typically, rickettsial bodies are smaller 

 than bacteria, the average size being about 

 half a micron. 



About 40 kinds of Rickettsia, mainly rod- 

 shaped and ball-shaped, have been observed, 

 particularly in the cells of the intestinal 

 wall and salivary glands of various lice, ticks, 

 and bedbugs. However, only two kinds seem 

 to be associated with common human di- 

 seases: (1) The typhus Rickettsia, which is 

 transmitted by lice; and (2) the Rocky Moun- 

 tain spotted fever Rickettsia, which is trans- 

 mitted by ticks. 



The antiquity of the Schizomycophyta as 

 a separately evolving group (Fig. 29-11) is 

 indicated not only by the primitive cellular 

 structure and the primitive reproductive 

 status, but also by the fact that bacterial 

 remnants appear to be present in the very 

 oldest fossil-bearing strata. 



The Myxomycophytes (Slime Molds). There 

 are about 500 species of these strange fungi. 

 Usually they are found living saprophyti- 

 cally mpon rotting wood, fruits, and other 

 masses of organic material. 



The vegetative body, or plasmodium, of 

 the slime mold typically consists of a lobu- 

 lated sheet of naked protoplasm (Fig. 31-7), 

 which creeps actively, like a giant amoeba, 

 over the surface of the food material. In some 

 species, the plasmodium may cover an area 

 of several square inches, but in others it may 

 be semimicroscopic. Sometimes the plasmo- 

 dium may be a multinucleate syncytium, but 

 in a few species it consists of a coordinated 

 mass of uninucleate cells, such as is shown in 

 Figure 2-22 (stage 1). 



For many years many slime molds were 

 thought of as animals. Then it was found 

 that each is capable of developing typical 

 multicellular sporangia (Fig. 31-8) in which 

 the cells possess cellulose walls. Each species 

 is best identified, in fact, by the sporangia. 

 These often are beautifully colored, in hues 

 of yellow, orange, brown, red, violet, or pur- 

 ple. However, the sporangia of Myxomyco- 

 phyta are seldom more than 5 mm in diam- 



