MORPHOLOGY AND RELATIONSHIPS OF MICROORGANISMS 35 



peritrichous, flagella on all sides. Old cells and cells transferred 

 from one medium to another are very apt to loose their flagella. 

 A young culture is most suitable for determination of motility. 

 True motility must not be confused with Brawnian movement, 

 which is a vibrating or oscillating motion of finely divided par- 

 ticles of almost any kind suspended in water, and visible when 

 examined under the microscope. This motion has not been 

 satisfactorily explained, but it is probably due to rapid changes 

 in surface tension of the liquid at the point of contact with the 

 particles. 



Reproduction in Bacteria. Multiplication in all bacteria is a 

 simple process. The cell commonly elongates or enlarges, a cell 

 wall develops across the middle, and the two cells separate. 

 This operation may occur with considerable rapidity. Some organ- 

 isms in favorable medium can grow to their full size and divide to 

 form two individuals in the course of twenty minutes to an hour. 

 If the organism could multiply in this geometric ratio for a short 

 time the number of resultant organisms would be practically 

 incalculable. For example, if a bacterium should divide every 

 half -hour, at the end of two days the progeny would be represented 

 by 2 s * 5 , a number having twenty-eight figures. Such rapid multi- 

 plication is never long continued, for food supply is never long 

 favorable, and waste products of the bacterial growth tend to 

 accumulate and diminish the rate. Nevertheless, the rapidity 

 of increase of bacteria accounts in a large measure for the consider- 

 able changes they bring about in a short time, as in the souring 

 of milk or invasion of the body in disease. 



Many bacteria also reproduce by means of spores. These 

 are of two types: endospores, produced inside the bacterial cell, 

 and arthrospores, consisting of entire differentiated cells. The 

 former are produced by certain bacilli and spirilla, the latter by 

 certain of the filamentous forms or trichobacteria. 



Endospores are formed by many bacilli, and possibly by 

 some spirilla. They are produced in response to some definite 

 stimulus, such as unfavorable conditions of the environment, 

 accumulation of waste products, or change in reaction of the 

 medium. The spore is essentially a portion of the protoplasm 

 of the cell which has expelled most. of its water and shrunken in 



