188 
METHODS IN PLANT HISTOLOGY 
older nets. Examine segments 4 or 5 mm. in length for the formation 
of young nets. The old nets may reach a length of 10 cm. Cultures 
are easily kept in the laboratory. If material which has been growing 
in a 0.5 to 1 per cent Knop’s solution be brought into tap water or 
pond water, zoospore formation may begin within 24 hours. Nets 
brought from the nutrient solution into a 1 to 4 per cent cane-sugar 
solution produce zoospores for 
mu Yi JJX 
m 
Jm 
bmv. 
c ? Tl 
a few days. 
Nets of all sizes should be 
selected for study. The seg¬ 
ments are coenocytic, and the 
nuclei of the older segments are 
hard to differentiate, except in 
stained preparations. Only one 
nucleus will be found in the 
young segments, but in the 
older segments the nuclei be¬ 
come very numerous. 
For fixing, use the special 
chromo-acetic-osmic formula. 
This should not produce plas- 
molysis in nets of any age. 
Iron-alum haematoxylin will 
differentiate the nuclei and 
pyrenoids, which may look 
alike with less precise stains. 
Use the Venetian turpentine 
method for mounting whole 
young nets and parts of older 
nets. Fine scissors should be used freely, because any attempt to 
arrange the material with needles will make it look as if the whole 
method of preparation were wrong. Parts of nets mounted whole are 
shown in Figure 41. 
For details of the formation of starch and for the finer details of 
the development of zoospores and gametes, Hydrodictyon should be 
imbedded and cut. 
Pleurococcus.—’This form, which is used everywhere as a labora¬ 
tory type of the unicellular green algae, is found on the bark of trees, 
where it is more abundant on the north side and near the ground. 
Fig. 41.— Hydrodictyon: A, part of young net 
with segments showing one pyrenoid and one or two 
nuclei; B, parts of three segments with nucleus, n, 
and pyrenoid, p, well differentiated; C, part of a still 
older segment with nuclei more numerous than the 
pyrenoids; D, part of a nearly mature segment with 
nuclei much more numerous than the pyrenoids. 
Fixed in the special chromo-acetic-osmic solution 
and stained in iron-alum haematoxylin. X600. 
