PROTOPLASM THROUGH THE WALLS OF VEGETABLE CELLS. 
827 
the only point of importance requiring attention is that the reagent should be allowed 
to act long enough. 
After treatment with Chlor. Zinc Iod. the section is well washed in water until the 
blue or brown colour (as the case may be) has disappeared. It is then placed for 
about a quarter of an hour in the picric-HoFMANN’s-blue solution, and after 
being well washed is mounted in glycerine or glycerine jelly. 
As before mentioned, the staining solution is made as follows : To 100 cub. cent, of 
strong alcohol (e.g. about 90 per cent, strength) is added an equal bulk of distilled 
water. The resulting solution is saturated with picric acid, and Hofmann’s blue is 
added, until the liquid is of a dark blue-green colour. It is then filtered. 
I used the method with Chlor. Zinc. Tod. almost entirely in my researches upon 
the structure of endosperm cells. I did not discover the picric-HoFMANN’s-blue 
modification until I had finished my work with pulvini. Thus, the results with 
pulvini rest mainly on the sulphuric acid modification. 
On the nature of the pit membrane. 
If a thin section of almost any tissue be treated with Chlor. Zinc Iod. it will be seen 
that the walls of almost every cell are distinctly pitted.* These pits are brought 
into prominence from the fact that whereas the thicker unpitted portions of the cell 
wall give a well-defined cellulose blue reaction, the thin pit-closing membranes stain but 
slightly, or in some cases apparently remain quite colourless, f Indeed, so common is 
this pitting, which the above-mentioned reaction demonstrates, that it would be a 
statement little short of the truth to say that every cell whatsoever, is pitted to a 
greater or less degree. Moreover, the closing membrane of the pit itself may also be 
pitted, as in the seed of Lupinus hirsulus, &c. As a rule, it is only in the case of thin 
walled cells that it is necessary to apply any reagent to bring this pitting into view, for 
the more the cell wall increases in thickness the more pronounced does the pitting 
become, until its appearance is at length so marked that we are accustomed to speak 
of it as a pitted cell par excellence. It is a point of special interest to note that 
* Cells of Strychnos, Dioscorea, and Tamus are notable exceptions. 
t Even in cases where an en face view of the pit will give the impression that no coloration has 
occurred, a transverse section will show that in reality it is slightly stained, and contrasts as markedly 
with the deep blue stain of the thick wall as the same blue staining of a young cambium cell does with 
that of the mature, full-grown cell which is subsequently produced. The pits in the parenchymatous cells 
of the petiole of Cycas revoluta are of special interest here. There are, as it were, two systems of pits. 
The larger, which are arranged in rows up the sides of the cell, face the intercellular spaces, and stain 
deep blue with iodine and Chlor. Zinc Iod. The smaller pits between the communicating cell walls, on 
the other hand, do not stain perceptibly when viewed from above. (See Plate 68, fig. 12.) De Bar? 
mentions pits of Cycas and Encephalartos which give a callus reaction. (See ‘ Verg. Anatomie,’ p. 125. 
See also Russow’s important paper, “ Ueber Tupfelbildung,” &c. Sitzber. der Dorpat Natur., 1882, 
pp. 350-389. Russow is inclined to think that no staining of the pit membrane occurs.) 
