Watson on Structure and Relation of the Plastid. 337 
or their supposed homologues in a series of plants, beginning 
with those low in the scale of evolution, and ascending to 
higher and more specialized types might prove suggestive. 
Collection and Fixation Methods.—As a simple and useful 
material, I was fortunate in discovering on some Nitella 
an abundant growth of a parasitic Coleochaete (C. soluta). 
Alcohol of 30 per cent. strength was used as a killing fluid, 
and the percentage was gradually increased up to 70 per 
cent. Similar treatment was adopted for Oscillarta, Clado- 
phora and Zygnema. Different fluids were used for Spiro- 
gyra. After repeated experiments, mercuric chloride 
proved most satisfactory. In the use of this agent, subse- 
quent staining was unnecessary, as sufficient differentiation 
was secured by the action of the mercuric salt. For Nitella, 
weak alcohol proved most satisfactory. Gardiner’s sulphuric- 
acid-iodine method for the demonstration of protoplasmic 
continuity proved of value, but a weak solution only of the 
acid was used for a short time. 
Anthoceros was killed in several fluids, the most satis- 
factory being Flemming’s compound and I per cent. chromic 
acid. Of the two chromic acid is preferable, as giving the 
clearest and least altered details. Among the mosses 
Funaria was chosen, and the best results were secured with 
I per cent. chromic acid. A variety of fluids was used on 
fern prothallia, and sulphuric-acid-iodine treatment gave 
very pleasing results. The delicate prothallia, after being 
killed in Flemming’s fluid and subsequently washed, were 
dipped quickly in dilute sulphuric acid solution, then washed 
and placed in iodine solution, rewashed, stained and mounted 
in I per cent. acetic acid. 
Of the Lycopodineae, Psilotwm was selected on account 
of its large nuclei and plastids. Various methods were used, 
Flemming’s fluid serving best, as a fixative. Serial micro- 
tome sections were also prepared in the usual manner. 
Amongst Monocotyledons, Cypripedium instgne and Dietfen- 
bachia were chosen, owing to the large size of the plastids. 
