128 BOTANY, 
to form a mycelium: that s always the result of the growth 
of the spores from the spore-cases. 
Practical Studies.—(a) Collect a quantity of Pond Scum and other 
aquatic vegetation, and preservein a dish of water, Mount portions 
of this material and search for desmids, using a 4-inch objective. 
Two-lobed or star-shaped desmids of a bright-green color may fre- 
quently be found. A large lunate desmid (Closlerium, Fig. 54) is 
often still more common. In the latter the clear protoplasm at each 
end is always streaming rapidly. 
(0) Collect a little of the brownish-yellow scum which in early 
spring gathers on the top of the water of brooks, ditches, and pools. 
Mount in water and examine with a high power. Hundreds of dia- 
toms may be seen moving rapidly across the field in every direction. 
In any such preparation many species of various shapes will be 
found. The prevailing form, however, is generally elongated and 
somewhat diamond-shaped. 
(c) Study in like manner the slimy coating upon dead leaves and 
twigs in water in the summer for diatoms. On some of these, very 
fine markings may be found. 
(@) Collect a quantity of bright-green Pond Scum which always 
abounds in shallow ponds and pools, and preserve in adish of water. 
Collect, also, some of tine same which has begun to turn yellow and 
brown. Upon mounting a bit of the first in water and examining 
with a high power, it will be found to consist of threads of cylindri- 
cal cells, each containing one or more spiral chlorophyll-bands (Spi- 
rogyra, Fig. 56) or star-shaped chlorophyll-budies (Zygnema). Upon 
mounting some of the second collecting. here and there the formation 
of resting spores may be observed. In all cases care must be taken 
not to mount too great u quantity of the material, nor to injure the 
plants by rough handling. 
(e) In the study of Black Moulds it is mostly necessary to make 
use of alcohol for freeing the specimens of air; afterwards they usu- 
ally require to be treated with a dilute alkali, as a weak solution of 
ammonia or potassic hydrate, which causes the hyphe to swell up to 
their original proportions. 
(f) Cut a lemon in two, and, squeezing ovt most of the juice, ex- 
pose the two halves to the air of an ordinary living-room or school- 
room for a few days, when various moulds will begin to develop. 
Under favorable circumstances Black Mould will predominate. It 
can be told by its dark color and the minute round black spore- 
cases on the ends of the erect hyphe. Mount a few hyphe (as di- 
rected in ¢ above) and examine hy phe, spore-cases, and spores. 
