218 Carter, — Studies on the Chloroplasts of Desmids. /. 
black, and entirely obscuring the cell-contents in whole mounts. Naturally 
it was quite impossible to get any idea of the structure of the chloroplast 
from such preparations, and consequently it was absolutely necessary to cut 
sections. The same phenomenon has been noticed, though not to the same 
extent, in a few other species of Cosmarium , and also in certain species of 
Closterium and Peniurn. On the whole, however, it is quite unusual for the 
cell-wall to absorb the stain in this way. 
In the case of those species of Closterium and Penium whose cell-walls 
seem to have such a marked affinity for haematoxylin, this is probably to be 
attributed to the presence of iron salts in the composition of the wall, 
which help to fix the stain. The same explanation, however, is not true in 
the case of Cosmarium biretum , since a test for iron in its cell-wall gave 
only negative results. In the latter species it is possibly the presence of 
considerable quantities of some other mineral salt, probably lime, which 
accounts for the deeply staining properties of the cell-wall. 
When the cell-wall in Closterium becomes very old and brown, the 
power of attracting the haematoxylin seems to be lost, and the cell-wall 
scarcely stains at all. This is possibly due to the fact that the iron probably 
becomes united with a large organic radical, in which complex form it is 
incapable of acting as a mordant. 
A large part of the work was done from fresh material collected chiefly 
in Sutton Park, Warwickshire, and in Devonshire, but a considerable 
number of species were investigated from material furnished by Professor 
G. S. West, which had been preserved for some time in formalin or potassium 
copper acetate. After well washing such material in water, the structure of 
the chloroplast was usually quite clear on staining. 
The investigation of a few species was carried out from some prepara- 
tions made by Dr. Liitkemuller, and very kindly lent by Professor G. S. 
West. The specimens had been stained in Delafield’s haematoxylin and 
were mounted in Venetian turpentine. 
The living material was always fixed as soon as possible after the 
collections had been made, because most species do not live long when 
removed from their natural habitat. The most satisfactory fixing reagent 
was found to be a hot solution of corrosive sublimate. Bouin’s picric- 
formol-acetic fixing solution always produces considerable shrinkage of the 
cell-contents, even when very much diluted, and with Flemming’s weaker 
solution the staining is rarely so satisfactory. The solution used consisted 
of corrosive sublimate, 3 grm. ; glacial acetic acid, 3 c.c. ; 50 per cent, 
alcohol, 1 00 c.c. 
The hot sublimate was removed from the material as soon as possible 
and replaced by clean 50 per cent, alcohol. After washing in several 
changes of alcohol the remaining sublimate was removed by the addition of 
a dilute alcoholic solution of iodine. The material was then taken either 
