ANATOMICAL TECHNIQUE 445 



Here the material available is frequently in a dried condition; and 

 although its protoplasmic structures have been obliterated, it is 

 still by reason of the durability of its essential organization emi- 

 nently worthy of investigation. Preservation of hard structures, 

 although not in general as necessary as hi the case of the softer 

 tissues, is often, however, a matter of prune importance. 



The skeletal structures of plants consist of more or less thick- 

 walled cells united by means of a cement substance which is pectic 

 in its nature. The process of preservation must be such as will 

 not bring about maceration of the hard elements by the dissolution 

 of the bonding material or middle lamella. Further, preservatives 

 which increase the resistance of the thick cell wall to sectioning 

 are in general undesirable. As a result of the conditions just indi- 

 cated, chromic acid and the salts of chromium cannot in general be 

 advantageously employed for the fixation of hard tissues in plants, 

 valuable as these reagents are in the study of protoplasmic struc- 

 tures. Where as perfect a preservation of the protoplasmic organi- 

 zation as possible is essential, and at the same time the avoidance 

 of undesirable changes in the thick cell wall and the cement sub- 

 stance are sought, alcoholic fluids are to be chosen. Excellent re- 

 agents in this connection are solutions of corrosive sublimate or picric 

 acid in alcohol of from 100 to 30 per cent strength. The strength of 

 the alcohol will depend on the nature of the material. In general, 

 those parts which by reason of the presence of a large proportion 

 of water are subject to shrinkage in strong alcohol should be fixed 

 in solutions containing a lower percentage, while harder and more 

 impenetrable tissues may be treated with a higher grade of alcohol. 

 In many instances it is advisable to use the alcohol without any 

 added reagent. This is quite generally the case with wood; for 

 the addition of corrosive sublimate, for example, is not of sufficient 

 advantage in connection with fixation to warrant its employment. 

 Corrosive sublimate, picric acid, or whatever killing reagent may 

 be employed in conjunction with the alcohol can conveniently be 

 used in saturated solution in whatever grade of alcohol is found most 

 advantageous. Where material is fixed in alcoholic fluids it must 

 be washed hi alcohol of corresponding or greater strength to remove 

 the excess of reagent . Corrosive sublimate forms a black precipitate 



