PRACTICAL PROCESSES IN VEGETABLE HISTOLOGY. 3 
glasses a drop of osmic acid properly diluted, and then,added to it 
distilled water. 
When, after a rest of twenty-four hours, we coloured the organ- 
isms in the latter glass by means of reagents, of which we shall 
speak later, we succeeded in showing the long filaments. This was, 
on the contrary, impossible with the organisms in the other glass ; 
a phenomenon which we attribute to a contraction of the filament 
in the latter case, and to an absence of contraction in the case of 
fixation by osmic acid.* 
Alcoholic solution of corrosive sublimate.—The effect of this solu- 
tion employed as a fixative is rapid, but of very short duration. It 
is used with advantage in studying aleurone. 
III. CONTRACTION. 
It is known that protoplasm, either free like the plasmodia of the 
Myxomycetes, or surrounded by a ternary membrane, as in multi- 
cellular plants, has at its periphery a hyaline layer, which remains 
in perfect continuity with the rest of the protoplasm, though dis- 
tinguished from it by its hyaline appearance, and a greater refrangi- 
bility. In the interior of the protoplasm a border of the same 
nature surrounds the vacuoles when there are any. It is this 
membranous layer which regulates the osmotic phenomena of the 
cell. It is very permeable to water, but very little so to the salts 
which are dissolved in it, so that on placing the cell in pure water 
or in water charged with salts, the capacity of the vacuoles is in- 
creased or diminished, the protoplasm is dilated or contracted. 
Amongst the substances which produce the latter effect must be 
mentioned solution of sugar, weak aqueous solution of chlorate of 
potash, d@Zu¢e alcohol, glycerin, and sulphuric acid. These agents 
contract the protoplasm to the extent of detaching it from the cell- 
membrane. At the same time they give it a consistency which en- 
ables it to be better distinguished. 
Solution of sugar, introduced gradually into the preparations, 
contracts the vacuoles without killing the protoplasm ; when the 
cell-sap is abundant, as in old cells of Spirogyra and Gdogonium, 
it may happen that the volume of the protoplasm will be reduced 
one-half. + 
Alcohol always kills the protoplasm. It contracts it only when 
dilute, the slower its action the more marked is its effect. Con- 
tracted by this agent, the protoplasmic substance becomes hard and 
resisting. 
Glycerin produces an analogous result, with this difference how- 
ever, that the protoplasm does not become so rigid. 
* Bull. Soc. Bot., iii. 22nd July, 1881, See this Journal, ii. (1882) p. 640. 
+ P. Van Tieghem, ‘ Traité de Botanique,’ p. 473. Paris, 1882, 
