6 PRACTICAL BOTANY 



of chromic acid, with the strength of the solution used, 

 from a few minutes to twenty-four hours or more. The 

 objects must then be washed thoroughly with water : 

 they are then to be placed in dilute methylated 

 spirit (50 per cent.), subsequently in stronger spirit 

 (70 per cent.), and finally in absolute alcohol or strong 

 methylated spirit, which must be changed so long 

 as any colour is still extracted from the objects. They 

 may be preserved in this for future use. 



When osmic acid is used, the fixing effect is pro- 

 duced much more rapidly ; in the case of simple 

 structures, such as unicellular or filamentous Algge, 

 a few minutes (515) generally suffices; in the case of 

 more complex structures, such as ovules, sporangia 

 growing points, &c., the object may be left in the acid 

 till it looks black on the exterior : it must be then well 

 washed with dilute alcohol (50 per cent.), and left in it 

 for some time, and be then removed to 70 per cent. 

 The sections are best mounted in dilute glycerine. In 

 some cases osmic acid produces an excessive blackening 

 of the cells, which can be removed by treatment with 

 chlorine- water. 



Of the hardening reagents above mentioned absolute 

 alcohol, methylated spirit, and picric or chromic acids 

 are those most generally used. 



The following is a useful method for preparing sea-weeds : to 

 a quantity of saturated solution of picric acid in sea-water add 

 three or four times its volume of sea-water, and treat the tissue 

 with it for ^ hr. to 2 hrs. : wash well with sea- water so as to re- 

 move the picric acid, and then treat successively with 30, 50, 70, 

 and 90 per cent, alcohol. 



It is advisable in cases in which the cell-walls tend to swell up 



