RAW CULTURE MEDIA. 



41 



The same media, and as many other sorts as are available, should be tested raw 

 in sterile, dry, Petri dishes 10 cm. Ijroad and 2 to 3 cm. deep. Fortliispurpo.se the 

 vegetables are prepared as fohows : First, select sound, clean specimens, especially 

 avoiding those which are cracked open ; next, scrul) their surface thoroughly uirder 

 the tap, and rinse them in distilled water. They are now soaked 5 or 10 minutes, 

 ore\-en 20 minutes, in 1:1000 water solution of mercuric chloride. The)' are then 

 removed and dried with or without a preliminary rinsing in sterile water. When 

 dry they are put on a sterile paper or plate, are cut into slices about 1.5 to 2 cm. 

 thick with a cold sterile knife, are picked up with sterile forceps, and are put into 



the Petri dishes in pairs 

 or fours, the cover being 

 immediately replaced. 

 Enough of the mercuric 

 chloride remains on the 

 surface to inhibit the 

 growth of any surface 

 organisms which have 

 not been killed outright, 

 and experience shows 

 that intruders are rarely 

 dragged over the cut 

 surface. The slices may 

 be inoculated at once 

 or after 36 hours incuba- 

 tion in a moist chamber 

 at 30° C, or 48 hours at 

 25'^ C. The latter course 

 is preferable. In either 

 case, half of the slices 

 in each dish must be 

 kept uninoculated for 

 comparison ffig. 36). This method is well adapted to the study of various soft-rot 

 organisms such as Bacillus carotovorits^ B. aroidcar^ B. oleracctr^ etc. 



PLANT JUICES (with and without THE ADDITION OF WATER). 



(i) Juice of the host-plant. (4) Prune-juice. 



{_') Piitato-Iiroth. {5) Orange-juice. 



(a) With sodium hydrate. (6) Coconut-water ( unsteamed).t 



(h) Without alkali. (7) Yellow corn-meal broth. 



(3) Cabbage or cauliflower broth. (S) Timothy-hay infusion. 



Fig. 36.- 



*FiG. 36. — Iris-rhizome-rot organism grown on raw carrot. The check piece is unchanged, the 

 inoculated piece has browned and softened. Incubated 4 days at aliout 23° C. 



f This is removed directly from the nut to sterile test-tubes by means of sterile pipettes, which 

 are useful in many ways. The pipettes should be dry-heated and kept from contamination in long, 

 narrow, covered tin bo.xes. These boxes may be cylindrical or rectangular, with an end cover. 

 The upper end of the pipette should be plugged firmly with cotton before sterilization, and this 

 should be pushed in a short distance beyond the end, so that when the finger is placed on the end 

 there will be an air-tight union. Scalpels, etc., should be sterilized in shorter bo.xes of the same 

 kind (fig. zy). 



