248 XIX. STRUCTURE OF THE THALLOPHYTA. 



bodies are stained in their natural position, and are very well 

 defined ; and we can also, by deeper focussing, distinguish the 

 nuclei. 



Reactions for Chlorophyll. We now examine another thread, 

 which we lay direct into a drop of picric alcohol, when the pyre- 

 noids are sharply defined in the yellowish brown stained cell- 

 contents. After a short time irregular brown bodies appear in the 

 chlorophyll plates, which proceed from the disorganised chloro- 

 phyll, and give us the hypochlorin or chlorophyllan reaction. 

 The same reaction will be obtained under the influence of other 

 acids. The chlorophyllan reaction may be taken as characteristic 

 of chlorophyll. Another reaction, likewise characteristic, is that 

 which arises upon treating chlorophyll with a saturated solution of 

 potash in water. The chlorophyll bodies are almost immediately 

 coloured yellow-brown, and, after at the most 1 to J hour, again 

 become green. The conversion of the yellow-brown into the green 

 colour takes place immediately upon wanning to boiling, or upon 

 addition of water, somewhat less rapidly upon addition of alcohol, 

 ether, or glycerine. 



Fixing and Staining Methods. In order to be able to study the 

 nuclei more closely, and to obtain a complete insight into their 

 distribution, we will bring other methods into use. This will, 

 besides, give us opportunity of learning some approved methods 

 of "fixing" and staining, which histological studies have, in recent 

 times, to thank for not unimportant advances. We place some 

 branches of the Cladophora in 1 per cent, chromic acid, other 

 small portions in concentrated picric acid, others in 1 per cent, 

 chrom-acetic acid (chromic acid O7 per cent., acetic acid 0*3 

 per cent.), and still others in chromosmium acetic acid (chromic 

 acid 0-5 per cent., osmic acid 0-2 per cent., glacial acetic acid 0'2 

 per cent.). In doing this, we must take care that the reagent is 

 at least 100 times the bulk of the object to be fixed. The chromic 

 acid, and the chrom-acetic acid, we allow to act for some hours, 

 even without disadvantage for twenty-four hours, the picric acid 

 for about twenty- four hours, or the chromosmium acetic acid 

 for about half an hour only. All these preparations must after- 

 wards be washed most carefully in running water ; they can, with 

 advantage, remain for up to twenty-four hours in water which 

 is frequently changed. Specially careful washing is required to 

 remove picric acid from the preparations when they have to be 

 stained with haematin-ammonia. The variously "fixed" and 



