Vol. 2] Gardner. — Cytological Studies in Cyanophyceae. 245 



The cells of the Principes and Margaritiferae groups of Oscil- 

 latoria are sufficiently short to allow one to see everything per- 

 fectly in properly stained isolated cells. 



If one wishes paraffin sections of plants belonging to other 

 genera, it is expedient to use potassium iodide-iodine as a killing 

 agent, for washing and dehydration can be accomplished at the 

 same time. Plants killed in this way can usually withstand being 

 dehydrated rapidly without shrinking. 



(c) KILLING AND FIXING AGENTS. 



Besides the killing agents just mentioned, viz., alcohol, picric 

 acid, and potassium iodide-iodine, the following were tried : — 



Flemming's weak and strong chrom-osmic-acetic solutions, 

 saturated aqueous solution of corrosive sublimate, 1 per cent, 

 chromic acid, Hermann's mixture, and iridium chloride. 



Potassium iodide-iodine seems to be perfectly satisfactory, 

 hence experimentation with many other solutions has not been 

 resorted to. Flemming's solutions are almost as good as the 

 iodine in the majority of cases, but do not have the advantage of 

 being so easily washed out, and do not dissolve the cross-walls. 



(d) STAINS AND STAINING. 



At the beginning of this investigation two points of technique 

 were specially considered. First, the best killing agents that 

 would produce practically uniform results when followed by a 

 variety of stains ; and second, the best differential stains that 

 would uniformly reveal homologous structures in a large enough 

 series of plants to form a basis for the interpretation of the cell 

 structure. 



Throughout the work, a Zeiss instrument, with Abbe con- 

 denser, 2 mm. apochromatic objective, and 8, 12, and 18 compen- 

 sating oculars has been used. A white light has been secured by 

 means of a special lamp, and, after being condensed by a large 

 globe of distilled water, has been modified at will by variously 

 colored glass disks. 



The following stains have been employed. They are all Grii- 

 bler's except those marked otherwise. 



