REAGENTS 29 



For a couple of years an extensive series of experiments has been 

 carried on with root-tips of Vicia faha, Allium cepa, and especially 

 with Trillium erectum. The stock chromo-acetic solution was tried 

 with the addition of from 1 to 10 c.c. of 1 per cent osmic acid, fixing 

 from 24 to 48 hours. While the solutions with the lower percentages of 

 osmic acid fixed fairly well, they proved decidedly inferior in staining, 

 especially with Haidenhain's iron-alum haematoxylin. Solutions with 

 9-10 c.c. of osmic acid are unnecessarily strong. The solution with 8 

 c.c. of osmic acid produces the best fixing, and the staining is brilliant, 

 especially with Haidenhain's haematoxylin. For some things, espe- 

 cially algae, the chromic acid may be reduced and the osmic acid in- 

 creased. Some suggestions are made in Part II, in connection with 

 various objects. 



PICRIC ACID 



Use a saturated solution in water or 70 per cent alcohol. One gram 

 of picric acid crystals will saturate about 75 c.c. of water or alcohol. 

 This reagent penetrates well and does not make the material brittle. 

 It is to be recommended when difficulty is anticipated in the cutting. 

 If used cold, the time varies from 1 to 24 hours, depending upon the 

 character of the tissue and size of the specimen. If used hot (85° C), 

 5 or 10 minutes will be sufficient. This fixing agent is used rather ex- 

 tensively by zoologists, especially for embryological work. Botanists 

 have not given it fair trial. Since it seems worthless for mitotic figures; 

 they have not made a thorough trial of it for other objects. It might 

 be worth while to try it for embryo sacs, free nuclear stages in the 

 female gametophytes of gymnosperms, and similar things for which 

 satisfactory fixing has not yet been devised. 



Material should be washed in 70 or 50 per cent alcohol. Water is 

 injurious, and some even go so far as to avoid aqueous stains, unless 

 the material has been thoroughly washed. The washing should be 

 continued until the material appears whitish and the alcohol no longer 

 becomes tinged with yellow. Picro-carmine gives its best results after 

 this reagent. Picric acid can be combined with various other fixing 

 agents, and so we have picro-sulphuric acid, picro-nitric acid, picro- 

 chromic acid, picro-chromic-sulphuric acid, picro-osmic acid, picro- 

 alcohol, and picro-corrosive sublimate. The picric acid in all mixtures 

 should be rather strong. 



