CULTIVATION BY STROKE AND PUNCTURE. 67 



very similar to each other, it is then far easier to 

 observe slight differences in their forms, which are 

 not marked enough in themselves to allow us to sepa- 

 rate the two varieties from one another. On this 

 account the culture characteristics are at present our 

 most important guide in recognising and classifying 

 the species. 



If it is desirable to preserve a successful and charac- 

 teristic stroke or puncture culture, the test-tube is 

 melted off a little below the cotton-wool plug. By 

 this means the culture, being shut off from the air, 

 remains unaltered for a long time,' and a collection of 

 such test-tubes may be set aside for comparison. 



It is often desirable to grow a culture upon a slide, 

 in order to be able to observe it better or to photo- 

 graph it, especially when a fairly high power is to be 

 used. For this purpose the glass of the culture test- 

 tube, with all its flaws and imperfections, is not at all 

 suitable. Stroke cultivations as well as plate cultiva- 

 tions can be made on the slide. For the former we 

 proceed in the following manner. The slide is steril- 

 ised, like other glass utensils, in the hot-air steriliser, 

 and when cool is placed in a sterilised dish, like those 

 used for ordinary plate cultivations. This dish is kept 

 as horizontal as possible, whilst some liquid gelatine 

 or agar-agar is taken with a sterilised pipette from a 

 test-tube and placed drop by drop upon the slide until 

 it is covered nearly up to its edges with an even layer 



