Chapter XIII — 137 — Vital Staining 



the granules quickly coalesce into a small number of large 

 globules (sometimes into a single globule) which come to 

 be closely appressed to the wall of the vacuole. Then they 

 diminish little by little in volume and disappear, while the 

 entire vacuole takes on a diffuse stain which later becomes more 

 pronounced. 



To summarize these phenomena briefly : there is a precipitation 

 of the vacuolar colloid, followed, when the stain is sufficiently con- 

 centrated, by a dissolution of the precipitate and by the homo- 

 geneous staining of the vacuolar sap. 



These phenomena may be compared with those described by 

 VON MoLLENDORFF in animal cells. He found, in staining with 

 neutral red the cells of the pronephros of amphibian tadpoles, that 

 basic dyes stain the fluid and acid inclusions of the cytoplasm (cor- 

 responding apparently to small vacuoles) and cause the precipita- 

 tion of the colloid of which they are composed, in the form of small 

 precipitates. Then, if an excess of the electropositive dye occurs 

 within the fluid electronegative inclusions, the precipitates formed 

 later in the fluid inclusions under the action of the dye are finally 

 dissolved and give to these inclusions a homogeneous color. To 

 explain this, voN Mollendorff supposes that the basic dyes pene- 

 trate the cytoplasm because of the lipides which it contains and in 

 which the dyes are soluble. Then they accumulate in the previ- 

 ously formed acid inclusions. There, the mixture of two colloids 

 of opposite signs, i.e., the electropositive dye and the electronega- 

 tive vacuolar colloid, produce precipitates. Then, when an excess 

 of dye is found in the vacuolar colloid, it communicates its charge 

 to the colloid, whereupon the precipitates are dissolved. 



It seems justifiable to apply this reasoning to the staining of 

 vacuoles of plant cells with vital dyes and we shall see, further on, 

 that the work we have carried out on yeasts, in collaboration with 

 Gautheret, seems indeed to confirm voN Mollendorff's hypo- 

 thesis. There is yet to be explained why the protoplasm remains un- 

 colored. Neutral red has an oxidation-reduction potential which 

 makes it unthinkable that it could be reduced in the cytoplasm, 

 and it must be supposed that the dye traverses the cytoplasm in a 

 degree of concentration which is too weak for its color to be per- 

 ceptible, and then later accumulates in the vacuole. 



The summary of vital staining which we have just given for 

 the vacuoles of yeasts with a dilute colloidal content applies to a 

 great number of cells, notably to the cells of the majority of fungi, 

 but it is not altogether general. In other cells, vital staining op- 

 erates in a different way. There are cases in which there is no 

 production of precipitates in those vacuoles which from the begin- 

 ning take a diffuse stain. There are other more frequent cases, in 

 which both precipitates and a diffuse staining of the vacuolar sap 

 occur at the same time. This diversity of behavior of the vacuoles 

 seems to depend on the nature of the colloids which they contain 

 and which are known to vary greatly from one type of cell to 

 another. 



