206 



METHODS IN PLANT HISTOLOGY 



alcohols, use a Petri dish and lay a slide over the material, and keep it 

 there until you imbed in paraffin. This will keep the filaments from 



Fig. 38. — Oscillatoria: photomicrograph from a paraffin section 3 /i in thickness and stained in 

 iron-alum haematoxylin. X373. 



spreading out too much, and you will be able to 

 get as much on one slide as you would be likely to 

 get on a dozen slides without such precaution. 



Oscillatoria, as it appears in section, is shown 

 in Figure 38. 



Tolypothrix. — This form occurs as small tufts, 

 either floating in stagnant water or attached to 

 plants and stones. Some species grow upon damp 

 ground. It furnishes an excellent example of 

 false branching (Fig. 39) . Like all small filamen- 

 tous algae, it may be dried on mica for herbarium 

 purposes. Venetian turpentine mounts and par- 

 affin sections are prepared as in Oscillatoria. 

 Tolypothrix is even better than Oscillatoria for a 

 study of the nucleus. 



Sc5rtonema is a similar form which is fairly 

 common. It is often found as a feltlike covering 

 on wet rocks. 



In staining forms like Tolypothrix and Scyto- 

 nema, 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 sections. Phloxine, with light green or 



Fig. 39. — Tolypothrix, 

 showing "false branch- 

 ing": h, heterocyst; c, 

 concave cell; b, end of 

 false branch with begin- 

 ning of new sheath. X620. 



