744 SUMMARY OF CURRENT RESEARCHES RELATING TO 



renders more visible, cliiefly by effect of contrast, rendering the 

 protoj)lasm wbich surrounds tbem soluble in the water. 



Carbolic acid. — E. Warming, whose interesting work on bacteria 

 and monads is well known, has found in carbolic acid a valuable agent 

 for rendering these little organisms transparent. 



Alcohol and Nitric acid. — We have obtained preparations of ex- 

 ireme thinness and of the greatest transparency * in the following 

 manner : — Place in a watch-glass the objects to be thinned (sections 

 of stems or roots) ; add to them alcohol of 36°, into which pour, drop 

 by drop, concentrated nitric acid until the red vapours of hyponitric 

 acid are disengaged. If the preparations are violently attacked, cover 

 the watch-glass with a small bell-glass, observing through it what 

 takes place in the liquid ; as soon as the preparations rise to the sur- 

 face of the mixture, raise the cover, and by means of two wooden 

 needles push them to the bottom of the glass. 



When there is no disengagement of red vapour at the normal 

 temperature, set fire to the alcohol in order to concentrate it 

 further, and warm the watch-glass on a piece of wire-gauze over a 

 gas-burner. 



Under these conditions, the cell-walls undergo a considerable 

 thinning, but all their contents disajipear. They become so delicate 

 that the difficulty is to remove them from the water in the evaporating 

 dish (into which the watch-glass has been emptied) to transfer them 

 into the glycerin of the slide. We attain this object by adding to 

 the still warm alcohol a little chloroform ; this treatment hardens the 

 preparations, which can then be transferred by means of little wooden 

 spatula into the glycerin, where they soon recover the same flexibility 

 as in the watch-glass. 



We have obtained better photographs of vegetable sections thus 

 prepared than with those obtained by other processes. 



Chromic acid. — According to Hohnel f this acid gives transparency 

 to tissues of a corky nature, such as cells of cork, epidermis, 

 cuticles, and the envelopes of pollen-grains, to the extent of making 

 details perfectly visible, which, without the aid of reagents, could not 

 have been seen. 



The solution of chromic acid admits of very different degrees of 

 concentration, the important point being that it should be free from 

 sulphuric acid. 



Calcium chlcnide. — When it is desired to give transparency to the 

 preparation withoiit thinning it, it may be very useful, especially if 

 the tissues are young, to have recourse to the process employed by 

 Treub,t and afterwards by Flahaut, § which consists, as described by 

 the latter author, " in placing the sections in a watch-glass or in a 

 small porcelain capsule with one or two drops of water ; the drop is 

 covered with a little dry calcium chloride in powder, and slowly 



* ' Recherclies sur I'appareil te'gumentairc des racines ' (8 pis. and 50 micro- 

 photographs). Paris, 1881. 



t "Ueber Kork," SB. Wiener Akad., 1877, 1 Abth. 



X ' Le m6ristome primitif de la racine des monocotylddones.' Leyde, 1876. 



§ "Rechcrchfis sur raccroissemeut terminal de la racine cUez les Phanc'ro- 

 gamcs," Ann. Sci. Nat., vi. (1878) p. 24. 



