BoT.— Vol. I.l PETRCE— MATURE OF LICHENS. 223 



For fixing I used : 



Chromic acid, 3 per cent., at room temperature. 



Chromic acid, i per cent., just below boiling temperature. 



Flemming's Chrom-osmic-acetic mixture, at room temperature. 



Corrosive sublimate, saturated solution in 35 per cent, alcohol, at room tem- 

 perature. 



Corrosive sublimate, saturated solution in 35 per cent, alcohol, just below 

 boiling. 



The last proved the most successful. Penetration at 

 room temperature is comparatively slow, owing to the large 

 amount of air enclosed in even wet lichens, whereas the 

 air is quickly driven out by the hot liquid. At room tem- 

 perature, the fragments treated with the fixing agent float, 

 and must be exhausted under the pump. Those treated 

 with the hot fixing-solution sink in it almost immediately. 

 There is, therefore, a saving of time, as well as greater 

 certainty of rapid penetration, if the fixing agent is applied 

 hot. Dehydration should not be carried on too rapidly, for 

 the gelatinous walls, especially of the outer hyphse, hold 

 the water. After complete dehydration, the material was 

 transferred from absolute alcohol to equal parts absolute 

 alcohol and xylol, then to pure xylol, and to this were 

 slowly added small fragments of paraffin melting at 55°. 

 At the same time the temperature was slowly raised to about 

 40°. Paraffin was then added to the point of saturation 

 and the xylol allowed to evaporate. When the xylol was 

 driven off, the specimens were transferred to melted hard 

 paraffin (melting-point about 59°), kept in this for two 

 hours to remove the last trace of xylol and to insure per- 

 fect penetration, then blocked, and sectioned at leisure. 



The gelatinous hyphae, soft and easily cut when wet, are 

 now hard and rather brittle. For very thin sections, and 

 these yield the most trustworthy results, the knife must be 

 even sharper than for most tissues. The best sections 

 were i yt* thick, made with a Zimmermann-Minot microtome, 

 and fastened to the slide, after floating and straightening on 

 warm water, by albumen fixative. Some were then stained 

 withCzoker's Alum-Cochineal and Bismark Brown. Some 



(2) May 29, 1899. 



