184 
METHODS IN PLANT HISTOLOGY 
Lutman has found that Closterium divides at night. If mitotic 
figures are wanted they are 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. 
The special chromo-acetic-osmic-acid solution is good for fixing. 
Stain in iron-alum haematoxylin and also in Magdala red and anilin 
blue. Follow the Venetian turpentine method, and in mounting 
put material from both stains on each slide. 
Textbooks describe “ stellate chromatophores ” in Zygnema. 
A good preparation should show that the chromatophores have 
an even outline, with no trace of a stellate form. The boundary 
between the chromatophore and the protoplasm—often of a stellate 
form—in which the chromatophore is imbedded, should be seen in 
well-fixed material with either of the foregoing stains. The large 
starch grains, extending from the pyrenoid almost to the border of 
the chromatophore, are better differentiated b^ the Magdala red 
and anilin blue; the nucleus and pyrenoid are better stained by the 
iron-alum haematoxylin. Careful staining should bring out the 
features shown in Figure 39. The chromatophores do not stain as 
readily as those of Spirogyra , and consequently it is necessary to use 
stronger stains or more prolonged periods. Use the Venetian turpen¬ 
tine method. 
For a detailed study, imbed in paraffin and cut thin sections. 
After washing in water, arrange the filaments so that most of them 
will have the same general direction; then, in running up through 
the alcohols, keep the filaments from spreading too much by placing 
a slide on the material. After imbedding, the material can be cut 
into blocks about a centimeter square. If sections thinner than 5 p 
are wanted, cut out smaller paraffin blocks. 
Spirogyra.—Probably no alga has been more studied by pupils, 
teachers, and investigators than Spirogyra. Nearly all of the 
numerous species belong to the low, quiet waters of ponds and 
ditches, where they often form large, flocculent green mats nearly 
