350 A TEXTBOOK OF THEORETICAL BOTANY 



and Pseudomonas fluorescens. One member of this group can live 

 on methylamine (CH3NH2) as its only organic material. 



2. Bacteria which require amino-acids as a source of Nitrogen. This is 



a very large group, which vary widely in their demands. Among 

 the amino-acids, tryptophane is the only one indispensable and 

 is thus the onlv one which they cannot synthesize. In this group 

 are Bacillus siibtilis, B. mycoides, and the organisms of the " enteric " 

 group, such as Salmonella typhosa. 



3. Bacteria which have complex nutritional needs, including in many 



cases special " accessory " factors, such as vitamins. This group 

 has the least powder of synthesis and includes a few specialized 

 parasites which will scarcely grow on artificial media. Here are 

 included Staphvlococcus, Corynebacterium diphtheriae and Hemo- 

 philus influenzae. 



Most Bacteria utilize Oxygen for respiration, as do higher plants, and 

 are thus aerobes, but Pasteur discovered that a number can dispense with 

 it and that a few, known as anaerobes, cannot grow when Oxygen is present. 

 Anaerobic respiration is closely allied to fermentation (see Volume III) and 

 involves a linked reaction between an oxidizable substance which gives up 

 Hydrogen and some easily reducible substance, which may be an organic 

 substance or even a nitrate, to which the Hydrogen is transferred. 



The yield of energy in this process of dehydrogenation is much less than 

 in aerobic respiration, and the anaerobic organisms have remained on a lower 

 plane of organization, but they are interesting biologically as their metabolism 

 is more primitive than that of the aerobic organisms. 



Reproduction 



Bacteria form one of the very few groups of organisms which appear 

 to be devoid of any sexual process. Observations have been made of cases 

 in which the two halves of a dividing cell re-fuse with each other and this 

 has been interpreted as a form of conjugation. Other and more doubtful 

 observations suggest that in some cases large numbers of cells may become 

 united into a free plasmatic mass, called a symplasm, from which new cells 

 are segregated anew. Nothing in this connection is accepted as certain. 



The universal method of multiplication is by binary fission, the cell 

 dividing into two equal parts by a transverse constriction of the cytoplasm, 

 followed by the deposition of a double cross- wall between the cells. The rate 

 of division is dependent on the temperature, but at 37° C. it occurs, on the 

 average, every half-hour. If every new cell formed divided again every 

 thirty minutes the number produced would increase in geometrical pro- 

 gression. Thus one individual would produce in this way in twenty-four 

 hours a total of 280 billion progeny, and if we assume that the volume of 

 each cell is 25 cu.ju,, the volume produced would be 7 litres of bacteria. This 

 theoretical rate of multiplication can only, fortunately, be maintained for a 

 short time and in the presence of unlimited food supplies. In practice the 



