Carter. — Studies on the Chlor op lasts of Desmids . /. 219 
down into water for staining, or up through absolute alcohol and xylol, and 
finally into paraffin for sectioning, the transference to the various liquids 
being made as gradual as possible. A centrifuge was always used for 
facilitating the removal of solutions, and thus it was possible to transfer the 
material without loss, and in a comparatively short time, through the 
numerous different strengths of solutions which were absolutely necessary in 
order to prevent the shrinkage of the cell-contents. In transferring from 
water to absolute alcohol and xylol, the material could be taken without 
shrinkage through solutions of strengths increasing successively by 10 per 
cent., being kept about 10 minutes in each one. It would have been quite 
impossible to get through so much material if it had been necessary to wait 
for the Desmids to settle by gravity each time the solution was changed. 
The most satisfactory stain was found to be Heidenhain’s iron-haema- 
toxylin. This always gives a perfect differentiation of the chloroplasts, the 
general cytoplasm staining very faintly or not at all. Material was trans- 
ferred from water to a 2 J per cent, solution of iron alum, and allowed to 
remain in this for 10 to 20 minutes. After rinsing in water it was stained 
for about 40 minutes in per cent, aqueous solution of haematoxylin. The 
proper differentiation of the cell- contents was obtained by washing out part 
of the stain in diluted mordant, the operation being controlled by watching 
a drop of the material under the microscope. After well washing in water 
the material was transferred gradually to absolute alcohol and xylol, and 
finally placed in very dilute Canada balsam, which was allowed to concen- 
trate slowly until it was of a consistency suitable for mounting. 
Venetian turpentine was also used instead of Canada balsam with 
equally good results. The material was transferred from 95 per cent, 
alcohol to the diluted turpentine, and allowed to concentrate in an atmo- 
sphere kept dry by means of fused calcium chloride. 
II. General Characters of the Chloroplasts. 
In the lower group of the Desmidiaceae the form of the chloroplasts is 
usually very much simpler than in the Placodermae, the only genera in 
which very complicated structure occurs being Netrium and Spirotaenia . 
Consequently the chloroplasts of these rather more primitive Desmids are 
very much better known. Thus the simple axile chloroplasts of Gonato- 
zygon and Mesotaenium , and the spirally-twisted bands of Genicular ia^ were 
figured by such early investigators as Nageli (1849) and de Bary (1858). 
The chloroplasts in two species of Spirotaenia were figured by Ralfs (1848), 
and the whole genus has since been thoroughly investigated by Liitkemuller 
(1895), whilst in the case of Roya the chloroplast has been figured by 
W. and G. S. West (1896). In all these genera the chloroplasts usually 
stretch from end to end of the cell, and the nucleus often occupies in con- 
