40 BACTERIA IN RELATION TO PLANT DISEASES. 



days. The addition of considerable ^^•ater 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, a la Roux, or bv 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. (s) Turnip. (g) Onion. (13) Brazilniits. 



(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 ma}- be extended almost indefinitely and are very useful for 

 making preliminary studies, inasmuch as the}' 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 sound and free from blemishes. They are now 

 to be washed thoroughh' 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. 

 C}'linders 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 the}- are finall}- 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 earh- stages of preparation and are steamed as soon as 

 they are placed in the tubes, /. c., 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 due regard 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, generalh' 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 

 hast)- conclusion, since different samples of potatoes, carrots, etc., vary somewhat 

 in composition and will at times give slightl}- \-arying results or even results which 

 seem to be contradictory, e.£., a brown pigment in some instances and not in others. 



