Sy 
STUDY OF COLONIES, 27 
GROWTH. 
The manner of growth and rapidity of growth at given temperatures in hanging 
drops and also on the margin of young colonies on plates of nutrient gelatin and 
agar of varying density should be determined. 
Frequently characteristic and interesting ar- 
rangements of the rods forming the surface 
layers of the colony, especially when it is 
young, may be discovered by means of a 
direct inspection of the colonies under low 
powers 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 layers of the 
surface colony. In directexamination the colonies 
should be viewed by reflected as well as by trans- 
mitted light. Drawings or photographs of surface rice nas 
colonies should be made under low or medium : é 
magnifications. By a little practice using Lister’s 
dilution method (’78, Bibliog., XVII), hanging- 
drops containing a single bacterium for study under 
the microscope may be obtained with Beyerinck’s 
capillary drop-flask (’91, Bibliog., X VII). 
CHEMOTROPISM. 
1 
5 
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 formerly attributed to chemical 
stimulus. Consult Jennings, “ Contributions to the study of the behavior of lower 
organisms,” Carnegie Institution of Washington, 1904, and especially 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. 
REACTION TO STAINS. 
Proper staining is a very 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 affinity, 
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 affinity for stains until acted 
*Fic, 25—Hand lens suitable for examining bacterial cultures. Zeiss aplanat magnifying six 
times. Three-~fourths 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. 
