158 



Methods in Plant Histology 



h 



and paraffin sections are prepared as in Oscillatoria. Tolypothrix is 



even better than Oscillatoria for a study of the nucleus. 



Scytonema is a similar form which is fairly common. It is often 



found as a felt-like covering on wet rocks. 



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. Anilin blue is 

 good for sections. It is equally good for Vene- 

 tian turpentine mounts, but is likely to over- 

 stain. You are likely to get good mounts with 

 less trouble if you use light green for the sheath. 

 Nostoc. Nostoc is a cosmopolitan form. It 

 occurs on damp earth or floating freely in water. 

 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 prepara- 

 tions look untidy, but they show that each 

 filament of the nodule has its own gelatinous 

 sheath. Small nodules may be stained in bulk 



i, -\r " - /^ij 



an ^ be got into Venetian turpentine. Crushed 

 under the cover, they make instructive prepa- 

 rations. 



Rivularia.--This form is readily found on the underside of the 

 leaves of water-lilies (Nuphar, Nymphaea, etc.), but is also abundant 

 on submerged 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- 



FIG. 29. Tolypo- 

 thrix, showing "false 

 branching " : h, heter- 

 ocyst; c, concave cell ; 

 6, end of false branch 

 with beginning of new 

 sheath. X620. 



