Rivett.—The Cytoplasm in the Cells of Alicularia scalaris. 211 
when the oil-body is intact owing to the refractive glimmer of the oil; when 
this is removed, the membrane shows up, probably with the addition of a 
slight precipitate ; that its surface would be changed by contact with the oil 
is extremely probable ; hence it absorbs a stain with more readiness than the 
remaining protoplasm. The formation of a second membrane on a second 
treatment with alcohol does not always take place, and is not nearly so con¬ 
spicuous as the first, having more the appearance of a deposit of oil in a cavity. 
I think we may safely conclude that the particular kind of membrane 
which is found around the oil-body does not present any argument against 
the idea that the bodies are vacuoles ; the membrane is either the limit of 
the undifferentiated cytoplasm, or the surface of the cytoplasm slightly 
changed by contact with the oil. It is not an organ of sufficiently definite 
morphological and physiological characters to warrant the use of the word 
‘ plastid ’ in connexion with the oil-bodies. 
This simple method of secretion into vacuoles agrees with the fact that 
the oil is a catabolic waste product. The production is unaffected by any 
cultural conditions; i. e. by variations in light, temperature, and nutritive 
materials. It is therefore quite easy to imagine the whole thing as a simple 
chemical process—the oil-bubbles being given off from the reagents, just as 
a bubble of gas emerges when an acid touches a metal. 
It may be objected that the regularity of form of the mature oil-bodies 
and the little variation of number in the cells produce a distinctness and 
individuality which argues against their description as spaces or vacuoles. 
This objection will certainly have weight with those who have observed the 
mature bodies only, but it cannot be maintained by those who have observed 
the emulsion of bubbles, out of which the oil-body arises. The constancy 
of form of the oil-vacuole is due to the equilibrium between the density of 
the oil and the density of the surrounding protoplasm. Under certain 
conditions a bubble can only take up one shape, and it is this which is seen 
in the constant form of the oil-bodies. 
Garjeanne (6) describes the methods which led him to conclude that the 
oil is secreted in a vacuole; by means of picric acid, which colours the 
cytoplasm yellow and leaves the spaces uncoloured, he discovered minute 
vacuoles into which the oil is poured. I have repeated his methods with 
similar results, but do not place much reliance upon them, since no other 
fixatives produce the same appearance, and it is obvious that picric acid so 
rapidly causes change and disorganization in the cytoplasm, that it may 
easily lead to false conclusions. 
The results of irrigating an entire leaf with 2 per cent, osmic acid while 
under the microscope, and also the investigation of entire leaves fixed in 
osmic for some hours, confirm the view that the oil is secreted in vacuoles. 
Both the first-formed drops and the mature oil-bodies stain homogeneously 
brown and stand out clearly from the surrounding protoplasm. If a series 
p 2 
