170 
METHODS IN PLANT HISTOLOGY 
In staining forms like Tolypothrix and Scytonema, which have a 
thick sheath, take care not to obscure the cell contents by staining 
the sheath too deeply. If the sheath is not stained at all, you may 
not be able to see the nature of the false branching. Iron-alum 
haematoxylin, with orange in clove oil for the sheath, is good for sec¬ 
tions. Magdala red, with light green for the 
sheath, is good for Venetian turpentine mounts. 
Nostoc.— Nostoc is a cosmopolitan form. It 
occurs on damp earth or floating freely in water. 
In a fruit can or a battery jar, Nostoc is easily 
kept year after year in the laboratory. Young 
specimens are generally in the form of gelatinous 
nodules, but in older specimens the form may 
be quite various. It is very easy to make 
sections, since the gelatinous matrix cuts well 
and holds the filaments together. Chromo- 
acetic acid is a good fixing agent. Stains which 
stain the gelatinous matrix make the preparations 
look untidy, but they show that each filament 
of the nodule has its own gelatinous sheath. 
Small nodules may be stained in bulk and be 
got into Venetian turpentine. Crushed under 
the cover, they make instructive preparations. 
Rivularia.—-This form is readily found on the 
underside of the leaves of water-lilies (. Nuphar , 
Nymphaea, etc.), but is also abundant on sub¬ 
merged leaves and stems of other plants. It occurs 
in the form of translucent, gelatinous nodules of 
various sizes. Chromo-acetic acid gives beautiful preparations, but 
good results can also be secured from formalin or picric-acid material. 
The most instructive preparations for morphological study can 
be obtained by the Venetian turpentine method. Stain in iron- 
haematoxylin and very lightly in erythrosin, the latter stain being 
used merely to outline the sheath. When ready for mounting, 
crush a small nodule under a cover-glass. The paraffin method is 
easily applied, since the gelatinous matrix keeps the filaments in 
place. Any form of similar habit may be prepared in the same way. 
Gloeotrichia — Gloeotrichia (Fig. 34), in its later stages, is a free- 
floating form. In earlier stages it is attached to various submersed 
Fig. 33.— Toly-pothrix f 
showing “false branch¬ 
ing”: h, heterocyst; c, con¬ 
cave cell; 6, end of false 
branch with beginning of 
new sheath. X620. 
