OSMIC ACID 323 



the latter down the neck. With a little practice just the desired 

 amount of fluid can be forced out by the heat of the hand. The 

 hand must not be placed on the bulb until the flask is inverted. 

 If it is desired to make cultures within the little flasks, snip off 

 the end of the capillary neck as before, and thrust a long platinum 

 needle, the end of which has been in contact with the source of 

 inoculation, down the neck into the fluid. Then withdraw the 

 needle and hermetically seal the neck in the flame. When cul 

 tures are to be made in the flasks, these should be only one-third 

 filled by the nutrient medium; there will then be sufficient air in 

 the flasks for the success of the culture after the flasks have been 

 inoculated and hermetically sealed. 



Pollen grains may be made to germinate in hanging-drops com- 

 posed of 100 parts of well-water, 3 to 30 parts of cane-sugar, and 

 1.5 parts of gelatine. This should be made as needed, or it may 

 be sterilized and kept indefinitely in the little flasks just described. 

 The amount of cane-sugar to give the best results varies with the 

 species of pollen, and can only be determined by experiment, but 

 3 parts will probably answer for most pollen grains. 



Spores of ferns may be made to germinate on pieces of flower- 

 pot which are kept half submerged in water and are covered by 

 a bell-jar. They should be set before a north window. They 

 should never be exposed to the direct light of the sun, since in 

 such a position the temperature under the bell-jar would become 

 very great. 



Osmic Acid. — The method of preparing a solution of osmic 

 acid and of its use in Flemming's fixative is given on page 259. 

 The vapor of osmic acid may be used as a fixative for very small 

 organisms. In order to accomplish this a drop of water con- 

 taining the organisms need only to be inverted over a bottle con- 

 taining a 2 per cent, solution of the acid. Osmic acid colors 

 ethereal and fatty oils from brown to black, but other organic 

 substances are also darkened by it; and as a test for oils it is not 

 absolutely reliable. Aleurone grains in sections of Ricinus which 

 have been freed from their oil by standing for a time in strong 

 alcohol may be stained brown, and the crystalloid and ground 



