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our Saccharomyces cerevisiae cannot utilise milk sugar as its source of 
carbon, then, is because it cannot split it up—z.¢., hydrolyse it—into 
the simpler assimilable sugars as it can cane-sugar. It can excrete 
the enzyme—invertase—necessary to alter the cane-sugar, but it can- 
not form the enzyme—lactase—which would hydrolyse the milk-sugar. 
If it could perform this preliminary splitting up. it would be perfectly 
well suited by feeding on the resulting glucose, as 1s proved by the 
fact that if we put our yeast into a solution containing milk-sugar to- 
gether with another fungus which caw break up this sugar, the former 
flourishes admirably. 
This theme is so important that I must risk taxing your patience 
by adducing one more example. Suppose in the nutritive solution 
for our fungus we substituted Starch for sugar. Here we should be 
putting a compound of C. H. and O. where again the proportions are 
not remarkably dissimilar*, and, as we know, starch is an admirable 
food-stuff even for many Fungi—e.g., LEurotiopsis, Aspergillus, 
Mucor, Pentcillium, etc.—we may be surprised to find that our fungus 
may starve apparently in the midst of plenty; here, again, it is found 
that this is simply because the yeast cannot activelyt alter the starch 
into assimilable sugars, for if we put in another fungus—.g., 
Eur otiopsis Gayoni or Aspergillus Oryzw, which excretes abundance 
of diastase and quickly hydrolyses the starch to glucose, the yeast then 
flourishes admirably. Numerous, other examples of the same kind 
can be given from among the Fungi, and there is no doubt as to the 
general application of the principles I have here illustrated by refer- 
ence to simple and well studied cases, for it must be remembered 
that wood-fungi, leaf-fungi, etc., all need sugars and similar bodies. 
It is evident, then, that in providing for the nutrition of a fungus, 
it by no means suffices to give it simply the necessary food-elements. 
The form in which these elements are combined in the food is a 
matter of fundamental importance. But experimental enquiry—and, 
as we see, the whole question is one of experiment—has resulted in 
the further discovery that great differences exist in the food require- 
ments of different Fungi, even within the limits of suitable food-media, 
and there can be no doubt that it is as important for an Agaric to 
obtain the right mixture as it is for a Mould to do so. 
Ordinary saprophytic mould-fungi, e¢.g., Mucor, Penicillium, As per- 
gillus, etc., will grow admirably on such a medium as the following : 
Sugar a ae ie e o> 15 grams, 
Peptone 5 £5 saat oS I 3 
Di-potassium phosphate (K H, PO,) O25 
Magnesium sulphate wie Pe 0°04 5, 
Calcium chloride — — — inh ia "0.02: 
Or, employing the same minerals, we may offer the carbon and 
nitrogen, etc., in the form of leucin and sugar, or ammonium tartrate 
and sugar, or peptone alone or leucin alone, or asparagin, or am- 
* The formula for starch is xC, H,, O,. 
+ I say actively because some varieties of yeast can slowly excrete a little diastase. 
