MINERAL ACIDS. 19 



mic acid, and, as the swelling proceeds slowly, the 

 dilute acid is useful for showing the stratification of 

 the wall. Only silicified and corky 1 layers resist 

 its action. Lignified cells are entirely dissolved. 

 Those containing suberin become very transparent 

 and nearly invisible ; but after the acid has been 

 removed by washing they usually reappear, though 

 the prolonged action of the reagent dissolves 

 them. 



In general, chromic acid is useful in the study 

 of the stratification of the cell-wall, starch-grains, 

 etc. It is also sometimes applicable to the fixation 

 of protoplasm \e. g. the plasmodia of Myxomy- 

 cetes], but must be used in a very dilute form for 

 this purpose. 



' PEROSMIC ACID. 



This poisonous and ill-smelling reagent, 2 which 

 is usually kept in the crystalline form in hermet- 

 ically-sealed tubes, and dissolved in water when 

 needed for use, has of late years been much em- 

 ployed in the investigation of the minute structure 



1 Hohnel: Ueber Kork. Sitzungsber. wiener Akad., 1871, Abth., I. 

 Hohnel differs here from Pollender : Chromsaure als Losungsmittel fur 

 Pollenin und Cutin. Bot. Zeitung, 1862, p. 385. 



2 Dippel: Das Mikroskop, L, p. 375. Strasburger : Befruchtung und 

 Zelltheilung, 1878; Studien iiber Protoplasma, 1876. Pfitzer: Die Bacil- 

 lariaceen. Hanstein's Bot. Abhandl., I., Heft 2, 1871, p. 33. Nageli : Das 

 Mikroskop, 1877, P- 476. Ranvier : Histologie, 1875, p. 55. Robin: 

 Microscope, 1877, P 22 - 



