174 



Methods in Plant Histology 



often contain some desmids. It frequently happens that a single 

 desirable desmid appears during examination of field collections. 

 In such a case, remove it with a fine pipette, and get it into a drop of 

 water on a clean slide, invert it over a bottle of 1 per cent osmic acid 

 for 2 minutes, leave the slide exposed to the air until almost all the 

 water has evaporated, and then add a drop of 10 per cent glycerin. 

 In a few hours (6 to 24) put on a cover and seal. It requires more 

 time, care, and patience than it is worth to attempt staining in such 

 a case. 



Sometimes desmids occur in great abundance. They may then 

 be treated like the filamentous algae, except that more care must 

 be taken not to lose them when changing fluids. The Venetian 



turpentine method, with Magdala red 



and anilin blue, will give beautiful 

 preparations. A deep stain with Mag- 

 dala red and a rather light stain with 

 anilin blue is better for the pyrenoids 

 and nucleus, while a light stain in the 

 red and a deep stain in blue is better 

 for the chromatophores. When the 

 material is sufficiently abundant, 

 paraffin sections may be made as 

 directed for Volvox. 



Lutman has found that Closterium divides at night. If mitotic 

 figures are wanted they are more likely to be obtained if the material 

 is fixed about midnight. 



Zygnema. -Zygnema is one of the commonest algae of the ponds, 

 swamps, and ditches. The mats are very slippery to the touch. 

 In the field it resembles Spirogyra, but is distinguished by the two 

 characteristic chromatophores which are readily seen with a good 

 pocket lens. Sometimes conjugation can be induced by bringing 

 the material into the laboratory and placing it in open jars with 

 plenty of water and not too much light. 



Iron-haematoxylin is a good stain for conjugating material. The 

 stain should be extracted until the four chromatophores become 

 distinct. The nuclei are small and inconspicuous. The chromato- 



FIG. 37. Zygnema: photomicro- 

 graph from a preparation stained 

 in iron -alum haematoxylin and 

 mounted in Venetian turpentine. 

 X700. 



