286 BOTANY: PRINCIPLES AND PROBLEMS 



size of bacteria makes a cytological study of them exceedingly 

 difficult. The cell wall rarely contains cellulose. One or more 

 cilia are found in many species, which thus have the power of 

 active motility. Various structural types are recognized by 

 bacteriologists, of which the commonest are the coccus form, 

 which is spherical; the bacterium or bacillus, which is rod-shaped, 

 and the spirillum, which is curved (Fig. 162). The only type of 

 reproduction known is simple cell-division, or fission, which is 

 unaccompanied by any of the complex phenomena found in the 

 higher plants, but which in the presence of an abundant food 

 supply may take place so frequently that a single cell will give 

 rise to milhons of individuals in a day's time. At the onset of 

 unfavorable conditions the protoplasm of the cell draws itself 

 together and produces a thick-walled spore which will germinate 

 whenever a favorable environment again ensues. Actively grow- 

 ing bacteria will ordinarily withstand relatively high tempera- 

 tures but their resting spores are still more resistant and will often 

 be found alive and able to germinate after hours of subjection to 

 boiling water. They can also survive extreme cold and dryness. 

 In their various characteristics, therefore, the bacteria (as we 

 have before noted) show much resemblance to the blue-green 

 algae. 



Although bacteria occur in countless myriads of individuals 

 and although their diverse activities make them almost omnipre- 

 sent in air, water, and soil, they are so small as to be quite invisible 

 and from a practical point of view would be entirely negligible 

 were it not for the profound effects which they produce. Their 

 importance in agriculture, particularly through their activity 

 in the soil and in dairy products ; their role as the chief agents of 

 fermentation and decay, and particularly their tremendous direct 

 interest to man as the cause of some of the worst and most preva- 

 lent of those diseases which afflict him and his domestic animals 

 and plants, have caused them to be studied with especial thorough- 

 ness and have established bacteriology as one of the most active 

 of the sciences. 



Our knowledge of bacteria dates only from the latter half of 

 the nineteenth century. Their existence was proven by that 

 great Frenchman, Louis Pasteur (Fig. 163), who labored for 

 years, meeting with the ridicule and antagonism which often 

 greets a new discovery, before he could convince his fellow 

 scientists that such minute objects were actually alive and were 



