FUNDAMENTAL CONSIDERATIONS 21 



has the advantage of mixing ' readily with 50 per cent, glycerin in 

 which the object should be placed before being mounted in this 

 medium. It should be warmed on a water bath before using and the 

 cover glass applied quickly after it is placed on the specimen. It 

 cools rapidly and constitutes the quickest and simplest means of 

 effecting a durable permanent mount. Its disadvantage is due 

 mainly to its jelly like consistency which is frequently responsible 

 for damaged mounts when the cover glass above the preparation is 

 too greatly strained. Canada balsam, slowly becomes solid, so 

 that the mount is exposed to no accident short of actual breakage. 

 Balsam has the disadvantage of being non-miscible with water, so 

 that before it can be used the object must be carefully dehydrated. 

 Even after this is done, and the object lying in absolute alcohol, an 

 oil must be used as an intermediate agent between alcohol and 

 balsam. 



2. Staining. For two reasons it is generally better to stain plant 

 tissues before mounting. Transparent tissues may become almost 

 invisible in glycerine, glycerin-gelatin, or balsam, and different 

 tissues take a stain differently. This being the case it becomes 

 possible to stain one tissue and not another, or one tissue with one 

 stain and another in the same section with a different stain, so that 

 the different parts may be brought out like areas on a colored map. 

 The most common stains are haematoxylin derived from logwood, and 

 various anilin stains safranin, fuchsin, eosin, iodine green, methyl- 

 green, malachite green, etc. 



METHOD FOR THE 'PREPARATION OF A CANADA BALSAM MOUNT 



1. Stain object with 0.5 per cent, solution of safranin or fuchsln 

 in 50 per cent, alcohol for from three to five minutes. 



2. Wash out excess of stain and further dehydrate with 70 per 

 cent, alcohol. 



3. Stain with 0.5 per cent, solution of methyl-green, or malachite 

 green, or iodine-green in 70 per cent, alcohol for twenty seconds or 

 longer, depending upon the nature of the material. 



4. Dehydrate and wash out excess of stain with 95 per cent, alcohol 

 for two minutes. 



