o 



06 BOTANICAL GAZETTE [october 



sufficiently gradual change is secured. When the paraffin is to be sup- 

 ported at the surface of the solvent, by buoyancy or other means, it 

 is better to add it in a round lump, because in this case slow diffusion 

 is desirable; but, when it is allowed to sink in the xylol, it should be 

 added in small fragments, since quick diffusion is desirable. 



Although in many laboratories xylol has taken the place of chloro- 

 form in work of this kind, the latter is still used by many technicians 

 because of certain advantages that it possesses over xylol. The object 

 in using either is not, as is often stated, to "clear" the specimen, but to 

 provide a medium for the introduction of the paraffin. Actual "clear- 

 ing," that is, the production of transparency, is of no benefit at all in 

 this stage of the process; in fact, it proves a disadvantage that can be 

 overcome only by staining in toto when only a few sections from a long 

 ribbon are to be selected for mounting. Chloroform has much less 

 tendency to produce transparency than has xylol. Moreover, as 

 DuGGAR^ has stated, a peculiarly undesirable optical effect is produced 

 permanently by xylol in some tissues. The use of chloroform makes 

 unnecessary, of course, any device for holding the paraffin at the top, 

 if this is thought necessary. 



The higher specific density of chloroform, which Land^ has men- 

 tioned as a factor to be considered, is aside from the question; it has 

 long been known that permeability and osmotic pressure, the things 

 which determine shrinkage, are not, as was once supposed, determined 

 by density when pure substances are being compared. Moreover, 

 neither chloroform nor xylol could "plasmolyze" a cell, as Land states; 

 at this stage of the process a cell is not subject to such life phenomena 

 as plasmolysis. 



The item of expense, which has caused the use of chloroform to be 

 discontinued in many places, and which is of special significance just 

 now, can be lessened by the easy and inexpensive recovery of about 

 one-half of the reagent used in the ordinary process. The waste chloro- 

 form, containing alcohol or other impurities, is thrown out of solution 

 with a large quantity of water, dried with calcium chloride, which 

 removes both alcohol and water, and distilled, the boiling point being 

 observed as a check on the purity of the product. — Paul Weatherwax, 

 Indiana University. 



^DuGGAR, B. M., Fungous diseases of plants. New York. 1909. p. 49. 

 " In a note added to an article by Mother, Bot. Gaz. 61:253. 1916. 



