76 STORRS AGRICULTURAL EXPERIMENT STATION. 
It seems clearly shown, therefore, that slight variations in 
composition of the medium do not produce great differences in 
the species studied in this paper. In the discussion of the 
relation of a mold to this gelatine it must be borne in mind 
that the same results might not follow the use of any other 
formula. | 
Potato agar was selected because of its use in many mycolog- 
ical laboratories. In the preparation of this medium, uniform 
composition can hardly be claimed. The following-process has 
been used in this work. ‘The potatoes are carefully washed, 
pared and sliced, then slowly heated for about two hours in 
approximately two volumes of water. At the close of the heat- 
ing the water is allowed to boil. "The whole is then: filtered 
through cloth, and commonly also through cotton; water is 
added to make up the losses of evaporation and filtering. To 
this is added one per cent. of shredded agar. It is then heated 
in the autoclave to 120° C. or higher, for from twenty to thirty 
minutes, when it may at once be put into tubes for use, or if 
cloudy it may be very quickly filtered through absorbent cot- 
ton, after which it should be quite clear. The uncertainties in 
the composition of this medium result from the differences in 
the potato extract itself and from the fact that the difficulties 
in filtering this extract take out a varying amount which is 
replaced with water. ‘Titration shows that this medium is 
nearly neutral (4-6 acid on Fuller’s scale) in cases tested to 
phenolphthalein, consequently it is used without neutralizing. 
Culture and study of the same species upon successive lots of 
this medium show that these differences in composition have 
little if any effect upon the morphology of the species studied. 
Petri dish cultures have been used continually because they 
admit of direct study under the microscope. Slanted test-tubes 
were found useful for stock cultures and for gross studies of 
physiological effects, but are of little value for comparative 
work. It is useless to attempt to get a correct idea of the nor- 
mal gross structure of these molds from fluid mounts. /’T’he 
extremely delicate hyphae are so tangled in such preparation 
as to give but very little idea of their ordinary appearance, 
while the chains of conidia break up immediately when placed 
in any fluid. Such mounts are useful and necessary to get at 
details of cell structure and cell relations, but in comparative 
