PHTCOPHTTA. 147 



These spores (called, also, conidia) may be seen as a whitish 

 halo surrounding the spot to which the fly (now dead) has 

 attached itself. Round and thick-walled resting spores 

 have been observed in some species, and may be studied in 

 the Grasshopper Fungus (Entomophthora grylli), which 

 destroys great numbers of grasshoppers every autumn. 



Practical Studies. — {a) Collect a quantity of pond-scum and other 

 aquatic vegetation, and preserve in a disli of water. Mount portions 

 of this material and search for desmids, using a -}-incli objective. 

 Two-lobed or star-shaped desmids of a bright-green color may fre- 

 quently be found. A large lunate desmid (Closterium, Fig. 71) is 

 often still more common. In the latter the clear protoplasm at each 

 end is always streaming rapidly. 



(6) 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. 



{d) Collect a quantity of bright-green pond-scum, which always 

 abounds in shallow ponds and pools, and preserve in a dish of water. 

 Collect, also, some of the 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. 73) or star-shaped chlorophyll-bodies (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 a 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 hyphae to swell up to 

 their original proportions. 



(/) Cut a lemon in two, and, squeezing out most of the juice, ex- 

 pose the two halves to the air of an ordinary living-room or school- 



