30 BACTERIA IN RELATION TO PLANT DISEASES. 



parasites is restrained by some nutrient gelatins which are neutral or only feebly 

 alkaline to litmus, it is advisable to add to a part of the stock more caustic soda than 

 is commonly used, /. t\, enough to render it neutral to phenolphthaleiii (strongly 

 alkaline to neutral litmus), especially if gelatin is selected as the first medium for 

 isolation experiments ; otherwise perplexing failures may result. 



(c) S/rcaks. Record the character of the streak, whether wet or dry, smooth, 

 wrinkled, or rough, thin or piled up, margin well defined or indistinct. Note also 

 whether the surface is ever iridescent, whether growths are sent down from the under 

 surface into the substratum, whether the streak spreads rapidly and widely over the 



surface or very slowly. The sur- 

 face behavior depends to some 

 extent on the motility of the 

 organism, on the amount of water 

 in the surface layers, /. e., whether 

 the slants are fresh or old, and on 

 the amount of gelatin in the me- 

 dium, which in temperate climates 

 should usually be 10 per cent, but 

 may be 15 or even 20 per cent. 

 By minimizing heat in prepara- 

 tion and by increasing the quan- 

 tity of gelatin to 20 or 30 per 

 cent a medium may be obtained 

 which will remain solid at 30 C. 

 Growth is less satisfactory, how- 

 ever, 011 such a dense medium, 

 or at least was in the few tests 

 made by the writer. Chester has 

 applied the ordinary botanical 

 terminology to the varying mar- 

 gins of colonies, etc., and has pub- 

 lished some useful figures ('01, 

 Bibliog., III). 



No substance used in the bac- 

 Fig. 28.* teriological laboratory is so uncer- 



tain and variable in its composition as gelatin. The gelatin from different factories 

 varies greatly and hardly any two batches from the same factory are alike. One glue 

 chemist has defined gelatin as "So per cent glue, 10 per cent dirt, and 10 per cent 

 doubt." It varies greatly in its melting point and power of setting, and in amount 

 of peptones and albumoses it may contain, which is sometimes large. It always con- 

 tains calcium salts and phosphates, which are often antiseptic, and the nature of which 

 varies according as hydrochloric or sulphurous acid has been used in its manufacture. 

 Formaldehyde is sometimes added to it, we are told ; and occasionally agar also, it is 



*Fic. 28. Nelson's photographic gelatin No. I. Recommended for bacteriological use. 



