ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 747 



tinguished from it by its hyaline appearance and a greater re- 

 frangibility. 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 increased 

 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, dilute 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 enables 

 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 Spirocjyra and (Edogonium, 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. 



Sulphuric acid acts in the same way, with more energy and rapidity. 

 It is important therefore to suspend the action as soon as the con- 

 traction has taken place. It would destroy the protoplasm if the 

 action were prolonged. 



Mineral acids generally behave in a similar way. 



These different substances, frequently employed in the examina- 

 tion of the protoplasm of the higher plants, can also be applied to 

 the study of the lower cryptogams which the simplicity of their 

 structure places at the confines of the two organic kingdoms. Dilute 

 alcohol, glycerin, and the mineral acids, by absorbing water, reduce 

 the bulk of the protoplasmic masses not surrounded by cell- 

 walls and destitute of vacuoles. We have used them success- 

 fully to determine the general contraction of the body of Monas 

 Okenii Ehr., and to show by that that this microbe, absolutely des- 

 titute of ternary envelope, must be removed from the bacteria and 

 associated with the nudo-flagellate organisms. 



Knowing the means of rendering the tissues transparent, of con- 

 tracting the organisms, and of fixing them in their forms, we must 

 now consider what kinds of histological elements or products of the 

 vegetable economy are capable of being revealed by means of crystalli- 

 zation, destruction, or colouring. In each of these three cases we shall 

 follow the inverse order to that which we have hitherto adopted; 

 instead of indicating, for each reagent, the different substances for 



* P. Van Tieghem, ' Traite de Botanique,' p. 473. Paris, 1882. 



