40 BACTERIA IN RELATION TO PLANT DISEASES. 
days. ‘The addition of considerable water enables one to keep the culture under 
observation for several months without danger from drying out if the cotton plugs 
are properly made. Drier culture media may also be used. If one wishes to do so, 
the potato or other substance may be lifted entirely out of the water by making a 
constriction in the lower part of the test tube, 4 la Roux, or by thrusting a wad of 
absorbent cotton into the bottom of the test tube before the potato is introduced. 
The writer has not found these methods necessary. In general, I prefer vegetable 
media which have been sterilized in the steamer rather than in the autoclave. 
The following are some of the vegetable substances recommended : 
(1) Potato. (5) Turnip. (9) Onion. (13) Brazilnuts. 
(2) Sweet potato. (6) Radish. (10) Banana. (14) Apple. 
(3) Carrot. (7) Salsify. (11) Coconut. (15) Pear or quince. 
(4) Sugar-beet. (8) Parsnip. (12) Peanuts. (16) Pineapple. 
These substances may be extended almost indefinitely and are very useful for 
making preliminary studies, inasmuch as they include many different kinds of 
chemical substances. The writer has used them for many years. ‘They should be 
prepared with great cleanliness, especially the roots, so as to avoid including resistant 
spores. Sterilization is an easy and simple process if these substrata are free from 
spores when the steaming begins. Roots and tubers should be selected with great 
care, only those being taken which are soundand free from blemishes. They are now 
to be washed thoroughly in tap water with scrubbing and then rinsed in distilled 
water. With clean hands and a clean knife they are then pared, with care to remove 
all black specks, and thrown into a beaker of distilled, filtered or boiled water. 
Cylinders of the proper size may now be punched with a clean cork-borer or cut 
with a clean sharp knife and, after the upper part has been slanted, are thrown 
into another beaker of distilled water, from which they are transferred to two others 
before they are finally put into the tubes. It is not necessary to soak them in water 
over night or in antiseptic solutions. ‘They will not brown by oxidization if they are 
kept under water during the early stages of preparation and are steamed as soon as 
they are placed in the tubes, z ¢., exposed-to the air. ‘They may be put into the 
tubes with clean fingers or by means of a pair of clean forceps. 
On these different media observe the nature, amount, and rapidity of growth 
(always with dueregard to the air-temperature, which should be recorded). Carefully 
determine whether there is any retardation of growth at first and, if so, to what it 
is due, so that more exact studies may be made subsequently in other media. Look 
for gas-bubbles, formation of acids and alkalies, formation of hydrogen sulphide, 
of crystals, of stains, of odors, destruction of starch, disappearance of the middle 
lamella, softening of cellulose, etc. For the first few days all cultures should be 
examined at least as often as once in 24 hours and, generally speaking, cultures 
should not be discarded until after the sixth or eighth week. These experiments 
should be repeated a* number of times and the student should avoid drawing a 
hasty conclusion, since different samples of potatoes, carrots, etc., vary somewhat 
in composition and will at times give slightly varying results or even results which 
seem to be contradictory, e. g., a brown pigment in some instances and not in others. 
