128 
(dextrose)—of quite different kinds before being absorbed. It tran- 
spired, for instance that the fungusSaccharomyces cannot absorb and 
digest cane-sugar itself until it has been thus broken up. This 
then was the first point, that the fungus has to so alter this par- 
ticular sugar into two others easily assimilable by it before it can 
begin feeding, and it is now known that it does this splitting— 
hydrolysis—by means of an enzyme which it excretes into the 
medium. 
The next point was that the carbon, hydrogen, oxygen and nitrogen 
and the mineral matters did not appear in the new fungus, built up 
by assimilation and growth, in anything like the same forms or pro- 
portions in which they were presented. For instance, no ammonia 
or tartaric acid or sugars were to be found in the fungus, or even 
in the unemployed food-medium at the end of the experiment. It 
took a long time to show what changes had occurred, and, put shortly, 
they were as follows. © Certain quantities of the carbon, hydrogen 
and oxygen, and most of the nitrogen had been built up into the 
protoplasm of the cell; some of the carbon, hydrogen and oxygen had 
been converted into the substance of the cell-walls, while a little had 
gone to form fats; and traces of the mineral matters K., P., S. and 
Mg:, had been retained in some way in the structure of the cells. 
But far larger proportions of the carbon, hydrogen and oxygen had 
not been used by the fungus at all—they had been converted into 
alcohol and carbon-dioxide, with traces of organic acids and glycerine 
in the liquid outside the fungus, and appear never to have gone into 
the fungus at all. ©The second and third points eventually estab- 
lished then were, that while none of the chemicals employed as food 
had been annihilated or lost, all had been altered; a certain pro- 
portion had gone to form new fungus-substance, but far more had been 
broken up and changed in the external medium. And this, of course, 
altered the medium itself as a source of food supply. The last of the 
four points here concerned was not thoroughly demonstrated until 
some time after. It is that although the sugars named are such ex- 
cellent media for supplying the most of the carbon, hydrogen and 
oxygen required by the Saccharomyces known as Beer-yeast, and there 
are one or two other kinds of sugar it can also digest and assimilate ; 
it starves if we attempt to feed it on other sugars. For instance, 
milk sugar (Lactose), though it contains carbon, hydrogen and oxygen 
in the same proportions as does cane-sugar,* is sweet and an excellent 
food, yet our yeast-fungus will starve if this is its only source of cat- 
bon. Now what explanation can we give of this ? 
It is not because Lactose is useless to all Fungi; because another 
species of yeast (Saccharomyces acidi-lactict is known which flourishes 
perfectly well in it. A clue is obtained when we recall the fact that 
our fungus cannot directly assimilate cane-sugar, and add that certain 
other simple Fungi—.g., Monilia candida—can do so. One reason 
* Strictly speaking 
-Cane sugar =C,, Hy, On, 
Lactose «= = C,,, H,, O11, + H, 0 


ee res 

