196 . BOTANICAL MICROTECHNIQUE 



brittle, and is especially good for firm structures that are to be sectioned 

 free-hand, such as the various parts of seed plants. 



Alcohol is the best all-round preservative. Other fluids have appeared 

 from time to time as rivals, as for example formalin, but they none of 

 them have supplanted it. It is somewhat uncertain whether denatured 

 alcohol, now on the market, will be just as good as the pure alcohol for 

 preservative purposes, and it should be used with some care until its effects 

 are known. 



178. Bringing fixed material into alcohol. Botanists are coming to depend 

 more and more upon fixed material even for general morphological studies, 

 since it is very little trouble and expense to fix in chrom-acetic acid, and 

 the superior results are worth the attention required. This is especially true 

 of type material of the thallophytes and bryophytes. 



Material fixed in chrom-acetic acid or in chrom-osmo-acetic acid (Flem- 

 ming's fluids) must be washed as described in Sec. 172, and then passed, or 

 "run up," through several grades of alcohol to 70% (or 85% if the material 

 is delicate), where it may rest indefinitely. It is well to begin with 15% 

 alcohol and pass successively through 25%, 35%, 50%, to 70%. Small objects 

 such as lily anthers will not require more than an hour in the lower grades. 

 They should remain, however, twice as long in the 35% and 50%. Larger 

 objects must remain from four to eight hours in each grade. The process 

 should be planned so that material is not left for so long a time as over night 

 in a grade of alcohol below 50%, and it should not remain in 50% longer than 

 over night. Generally the entire process can be finished in one day. The 

 grades of alcohol are made from 95%, which for general purposes is regarded 

 as being pure. 



Material fixed in fluids based on alcohol, as for example Carnoy's fluid, 

 should be passed directly into a grade of alcohol corresponding to that in the 

 fixing fluid. 



179. Formalin, Much was expected of formalin when it appeared a num- 

 ber of years ago. The most important claims have not been fulfilled. It will 

 not preserve the green color of plants in the light, and shades of red, blue, 

 and brown are generally modified after a few months. Unless the tissue is 

 firm it is apt sooner or later to soften or macerate ; this is especially true of 

 the lower plants. Finally, formalin is intensely disagreeable to work with on 

 account of its effect on the nose and eyes. 



Formalin, which is about 40% formaldehyde, is added to water to make a 

 2-5% solution. It is convenient to carry, since a small quantity will make 

 many quarts of the preserving fluid. If material is to be used within a short 

 time, formalin will prove satisfactory. Material may also be transferred from 

 formalin to alcohol, being carried up through the grades. However, its ad- 

 vantages are rather doubtful when chrom-acetic acid and alcohol are at hand. 



