THE FOOD OF PLANTS. 



99 



nature. To obtain it with certainty, we pour a very small quantity 

 of water over some hay, let it stand for four hours at a tempera- 

 ture of 36 C., pour oft' the fluid, without filtering, and dilute it 

 until its specific gravity is 1*004. If the decoction is too acid, we 

 now neutralise it with Sodium carbonate, then transfer to a flask 

 of 800 c.c. capacity, plug this with cotton wool, and heat the fluid 

 to boiling. It is gently boiled for an hour, and then put into the 

 thermostat at a temperature of 36 C. In the course of one or two 

 days a grey skin forms on the hay extract, which is composed of 

 the zooglcea of the bacillus. The spores of the organism have sur- 

 vived the boiling, other bacteria which were present in the decoc- 

 tion have been destroyed, and we obtain in this way a pure culture 





FIG. 28. Bacillus subtilis. A, the skin ; B, swarming rodlets ; C, spore formation. A, 

 magn. 500; B, magn. 1000; C, magn. 800. (After Strasburger.) 



of the hay fungus. The skin is a jelly, in which are present 

 large numbers of threads running parallel to one another, and 

 composed of rodlets. In order to bring out very clearly the rod- 

 lets of which the threads are composed, we stain them, a plan 

 often adopted in investigating bacteria. A small quantity of the 

 bacterium-containing fluid is placed on a cover-glass, spread out 

 over it and exposed to the air for some time to dry. We now 

 7*apidly draw the cover-glass several times through a spirit flame, 

 keeping the side covered with bacteria upwards, spread out on it 

 a drop of aqueous methyl violet or fuchsin solution (best prepared 

 by carefully adding a small quantity of the alcoholic solution of 

 the pigment to distilled water), and float it for twenty or thirty 

 minutes on distilled water, preparation downwards, afterwards 



