i9*o] BRIEFER ARTICLES • 83 



acid. Pure acid may be used if it is desired to hurry the process, but 

 great care should be taken and the material should be examined often 

 and removed as soon as it cuts easily with a Gillette razor blade. 

 Rhizomes of Osmunda and similar material, possessing very hard sheath- 

 ing leaf bases, may remain in 50 per cent acid several months without 

 apparent injury to other than the most fragile parenchyma tissues. 

 Blocks of Dioon spinulosum 1-2 cm. square were treated with 50 per 

 cent acid 3-6 weeks with gratifying results. Welwitschia, the delicate 

 parenchyma tissue of which is crowded with rigid spicular cells of great 

 size, after treating with 50 per cent hydrofluoric acid and imbedding in 

 paraffin, can be sectioned without difficulty. Rhipogonum scandens, a 

 New Zealand liana, impossible to section by ordinary methods due to 



f the extensive amount of sclerenchymatous tissue distributed through 



the stem, especially surrounding the scattered bundles, sectioned with 

 perfect ease after immersion for one week in full strength hydrofluoric 

 acid. Non-homogeneous material, such as corn stem, usually difficult 



I to section, especially after it attains a diameter of 1 .5-2.5 cm., because 



of the rigidity of the bundles and the delicate character of the paren- 

 chyma, was treated with a 25 per cent solution of hydrofluoric acid for 



[ one week, and sections 15-20 /x in thickness were easily cut from 52 C. 



( paraffin. In order to minimize the time of heating of such material in 



the paraffin bath during the infiltration process, blocks should not 

 average more than 1-1 . 5 cm. in thickness. The leaves and stem of 

 wheat, oats, and other cereals also contain more or less silica, which 

 makes them very refractory objects to cut, and accounts for the dif- 



| ficulty in obtaining sections of uredospores and teleutospores of 



Puccinia graminis. Immersion of these leaves in 10 per cent solution 

 of hydrofluoric acid for a few days, possibly a week, should remove 

 much of the silica without appreciable injury either to the cell walls or 

 cell contents. After removal from the softening medium, the material 

 should be thoroughly washed in running water to remove all traces of 

 the reagent and then placed in 60 per cent alcohol. 



Dehydration and clearing. — In passing through the alcohol and 

 xylol series in the process of dehydration and clearing, the time required 

 for each stage ranges from 12 hours for each of the 60, 70, 80, and 95 

 grades of alcohol, to 24 hours for the absolute alcohol and each of the 

 absolute alcohol and xylol series. Four grades, 25, 50, 75, and 100 

 per cent xylol, are generally sufficient. Any air or gases remaining in 

 the tissues should be removed by means of a vacuum pump while the 

 woody material is in pure xylol before the addition of the paraffin. 



