102 BACTERIAL DISEASES OF 'PLANTS 



There should be enough water added so that the culture will 

 remain moist for a number of weeks but not enough so that the 

 slant surface is covered. For isolation purposes, by means of 

 parallel strokes, the slant should be long and the amount of 

 water such that it will in no case flood over the slant surface. 

 Merely for the study of the organism on a particular medium 

 this is not so necessary. When the tubes have been filled and 

 plugged they must be steamed on 3 consecutive days, for 15 

 minutes each time. In default of a steamer they may be 

 stood upright in a tea-kettle and boiled. Formerly we had much 

 trouble in this laboratory in sterilizing potatoes and similar 

 substances derived from roots, i.e., after three steamings the 

 cultures would often develop a wrinkled white or gray white 

 bacterial growth. This growth came from spore-bearing bac- 

 teria lodged on the surface of the potato tuber and introduced 

 into our tubes through defective technic in their preparation, i.e., 

 we were not careful enough in paring and handling the potatoes 

 and other vegetables used, the surface of which almost always 

 contains resistant spore-bearing bacteria. We are now very 

 particular in the preparation of these media not to contaminate 

 either the pared portions or the cylinders punched from the 

 same. For that reason we rarely handle them with our fingers 

 in the last stages, but with forceps, and before putting them into 

 the tubes we give them a final rinsing in beakers of distilled 

 water, and now generally we autoclave such media. To avoid 

 erroneous conclusions it is well to let the sterilized medium 

 remain on the shelf some days before using it, that the unkilled 

 spores of fungi and bacteria may germinate, and always, of 

 course, each tube should be critically inspected before it is in- 

 oculated. In some laboratories it is difficult to keep media 

 sterile because Penicilliums and other common fungi have been 

 allowed to fruit freely in the rooms until the whole laboratory 

 has become an unclean place. Such laboratories are a disgrace 

 to the profession. The first thing is to have a general cleaning 

 up, and then to stay clean. In 1917 we had much trouble for 

 some weeks from a spore-bearing, very heat-resistant and acid- 

 resistant white schizomycete introduced from the Middle West 

 on dirty culture dishes. 



