166 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



which are difficult to see except under the highest power of the 

 microscope. 



The cell wall of a bacterium is usually considered as a selectively 

 permeable membrane, very delicate, and secreted by the cytoplasm. 

 A gelatinous capsule may be formed by some bacteria, so that groujis 

 of them clump together in masses. Although pigments are often 

 present, bacteria contain no chlorophyll, and consequently most of 

 them are dependent on other organisms for their food. They feed 

 both on living and dead organisms, using not only organic foodstuffs, 

 such as starches, sugars, and proteins, but even leather or wood. 

 Since their food must be liquid in order to be absorbed, they form 

 digestive enzymes within the cell which exude to digest the food out- 

 side of the cell body. 



In addition to these foods, bacteria need certain mineral salts that 

 are found in protoplasm, water, and nitrogen in a usable form. Not 

 all bacteria are capable of nitrogen fixing, but many obtain their 

 supply of nitrogen for tissue building as green plants do, in the form of 

 compounds of ammonia or nitric acid. 



The chromatin material is scattered through the cell, there being 

 no distinct nucleus in most bacteria. Bacteria need moisture, a favor- 

 able temperature, and food, in order to grow. Under favorable con- 

 ditions they multiply with great rapidity by simple fission. Under 

 unfavorable conditions, many bacterial cells can contract, lose con- 

 siderable water, and form resistant coats, thus making spores, which 

 can stand extreme conditions of dryness and temperature. While 

 bacteria are usually killed by heating to 100° C, some spores can 

 withstand this temperature for long periods. 



Functional Differences between Plant and Animal Cells 



A comparison of the several types of unicellular organisms described 

 might seem at first to show hard and fast distinctions between plant 

 and animal cells. Although chlorophyll is associated with plants, it is 

 sometimes found in borderline animals, while many plants, such as 

 the fungi and bacteria, lack chlorophyll. Locomotion is not exclu- 

 sively an animal characteristic. Some animal cells, as Vorticella, are 

 fixed during a part of their life history, while many unicellular plants 

 move freely through the water. Other plants, although fixed for part 

 of their lives, produce sex cells that are motile in water. The greatest 

 difference exists in methods of nutrition. In the green plant cell, for 

 instance, food substances are made inside the cell in the presence of 



