ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 639 



strands. Picric acid has also another valuable property in tending to 

 prevent the staining of cellulose by dyes which, although possessing 

 an especial affinity for protoplasm, will stain the cell-wall also unless 

 some such restraining reagent be used. The sections are first stained 

 with iodine and mounted in cblor-zinc-iod. If the material is 

 favourable, something may then be seen of the threads. After being 

 exposed to the action of the chlor-zinc-iod for about 12 hours, 

 the sections are well washed, stained with picric-Hoffmann's-blue, 

 washed again in water, and finally mounted in glycerin, or, better still, 

 placed in alcohol, first dilute and at length absolute, cleared with oil of 

 cloves, and mounted in Canada balsam. In those cases where the tissue 

 swells raj)idly under the action of the reagent, as in the endosperm of 

 Strychnos nux-vomica, Bauhinia, and Tamus, the action need not be so 

 prolonged, and the excessive swelling must be prevented by the use of 

 alcoholic iodine at the outset, and in a similar manner it may be 

 washed with alcohol instead of with water, otherwise the threads will 

 be so displaced or altered as to be almost or entirely invisible. 



As regards the management of the reagents, and the length of time 

 they must be allowed to act to obtain a satisfactory result, the mani- 

 pulation must be varied to a certain extent to suit the requirements of 

 the various kinds of tissue, according as it is thick- or thin-walled, 

 easily swollen or only with difficulty. The use of sulphuric acid is 

 attended with a much greater amount of difficulty ; for if it is 

 allowed to act for too short a time, the cell-wall will not be sufficiently 

 swollen ; while if the treatment is prolonged, the middle lamelleB of 

 the walls are liable to swell and at the same time stain, and will 

 then hinder all successful observation of the threads which may traverse 

 their substance. Upon still further action the protoplasm itself com- 

 mences to be attacked. With chlor-zinc-iod, on the other hand, 

 where the action is much more regulated and gradual, but little pre- 

 caution, as to length of time need be observed. Besides the difficulty 

 of regulating its action, there are still other and grave objections to 

 the use of sulphuric acid. One of these is that, no matter how care- 

 fully the acid is added to the tissue, and no matter how quickly the 

 washing in water is accomplished, there will be a very considerable 

 evolution of heat attending the hydration of the acid, which is liable 

 to accelerate its action and to cause very grave changes in such delicate 

 structures as fine protoplasmic threads traversing the cell- wall. The 

 folding up and general displacement of the tissue consequent upon the 

 action of such a violent reagent also greatly increases the already 

 existing complications which attend all observations connected with 

 minute histology. 



For these reasons, while sulphuric acid is a very valuable reagent, 

 both for swelling up resistent tissues on which chlor-zinc-iod has 

 but little action, and for demonstrating in an unusually clear way the 

 remarkable manner in which the apices of the protoplasmic processes, 

 entering the pits, cling to the pit-closing membrane, it is, on the 

 whole, the less satisfactory of the two, and the phenomena resulting 

 from its action can only be rightly interpreted in the light of the 

 more certain results obtained by the use of chlor-zinc-iod. For all 



