STUDY OF COLONIES. 



27 



GROWTH. 



The manner of growth and rapidit)- of growth at given temperatures in hanging 

 drops and also on the margin of )-oung colonies on plates of nutrient gelatin and 



agar of varying densit)' should be determined. 

 Frequently characteristic and interesting ar- 

 rangements of the rods forming the surface 

 la\'ers of the colony, especially when it is 

 young, may be discovered by means of a 

 direct inspection of the colonies under low 

 ]iowers of the microscope or by means of 

 cover-glass impressions. Covers are carefully 

 placed on the colony, removed, dried, flamed, 

 and stained. There are also often curious 

 Fig- 25.* arrangements of the deeper laj'crs of the 



surface colony. In direct examination the colonies 

 should be \-iewed by reflected as well as by trans- 

 mitted light. Drawings or photographs of surface 

 colonies should be made under low or medium 

 magnifications. By a little practice using Lister's 

 dilution method ('78, Bibliog., XVII), hanging- 

 drops containing a single bacterium for stud)- under 

 the microscope may be obtained with Beyerinck's 

 capillary drop-flask ('91, Biljliog., XVII). 



CHEMOTROPISM. 



Fig. 26.t 



On the general subject of chemotropism, see papers by Pfeffer, Miyoshi, 

 Jennings, Buller, Rothert, etc. Jennings maintains that contact irritation inducing 

 motor reflex is responsible for movements which were fonnerly attributed to chemical 

 stimulus. Consult Jennings, " Contributions to the study of the behavior of lower 

 organisms," Carnegie Institution of Washington, 1904, and especiall)- Jennings and 

 Crosby, "The manner in which bacteria react to stimuli, especially to chemical 

 stimuli," Am. Jour. Physiol, Vol. VI., pp. 31-37, and Jour. Roy. Mic. Soc, 1902, 

 p. 88. Spirillum volutans was used in the tests. 



RE.'^CTION TO STAINS. 



Proper staining is a vers' important part of the study of bacteria. Its founda- 

 tion principle is the fact that the bacteria, in a living vegetative condition, all show 

 a great affinity for the basic anilin dyes. Spores ordinarily show no such aflfinity, 

 but may be made to take up stains by acting on them with strong acids or alkalis 

 or by heating them very hot. Flagella also show no afl!inity for stains until acted 



*FiG. 25. — Hand Jens suitable for examining bacterial cultures. Zeiss aplanat magnifyino- six 

 times. Tihree-fourt'hs natural size. 



tFig. 26. — Zeiss swing-cover aplanat magnifying six times. This is now sent out in a neat little 

 chamois-skin purse. About two-thirds natural size. 



