Dehydrafion for Embedding 23 



more or less prolonged washing of the tissues in water; other fluids 

 require no washing, and dehydration is begun directly alter killing 

 or after a brief rinsing in water. Begin dehydration with a dehydrant 

 having approximately the same percentage of water as the killing 

 or storage fluid. For example, after FAA, begin dehydration in 50% 

 alcohol. After weak chrome-acetic or the weaker Craf type formula, 

 such as I and II, begin in 5 or 10% alcohol. The stronger formulas 

 such as III, IV, and V, which have greater hardening action, make 

 it possible to begin dehydration in 20 or 30^; alcohol. When using 

 the Craf V (Cornell) formula for routine chromosome counts, 

 root tips may be transferred directly from the killing fluid into 75% 

 alcohol. After Bouins fluid, begin with 50% for firm subjects and 

 20% for delicate materials. 



Solutions in the dehydrating series are changed by decanting the 

 liquid from the tissues and promptly flooding the material with a 

 generous volume of the solution next in the series. A piece of fine 

 brass-wire screen or a layer of cheesecloth is used to retain materials 

 that tend to float out of the bottle. The volatility of the solutions 

 high in the series demands speed in making the change to avoid 

 drying of the tissues. The material should not be permitted to become 

 dry even for an instant at any stage in the process. Never drain the 

 fluid from several specimen bottles, and then look on the shelf for 

 the next reagent, only to find that the bottle is empty. 



The interval in each of the solutions in the series depends on the 

 size of the pieces, the nature of the material, and the solubility of 

 the residual reagents left in the tissues. For root tips or small pieces 

 of leaf use 30-min. intervals up to 70%. After a picric acid formula 

 make each interval 1 hr. For twigs killed in FAA use 4- to 8-hr. 

 intervals up to 709; . For large blocks of wood the interval should 

 be about 12 hr. Beginning with 70%, double the previous interval 

 for each grade. Change the cork for a thoroughly dry one when first 

 changing to 100% alcohol. Make three changes of anhydrous alcohol. 

 Plan the timing of the dehydration series so that the series is stopped 

 at 70% for storage until you can resume the process. 



Some workers are inclined to make an imnecessary ritual of the 

 time element in dehydration. It is recognized that drastic changes of 

 concentration bring about shrinkage of protoplasm and distortion of 

 cells. Long intervals in low concentrations of dehydrating fluid, or 

 long washing in water, tend to make tissues soft and promote dis- 

 organization. ^Long exposure to high concentrations or anhydrous 

 reagents shrinks tissues and causes brittleness. With these general 



