l and do not elaborate starch. The quantity of trehalose in such
plants reaches a maximum just before spore formation begins. Since it is
manufactured in the absence of chlorophyll, its formation must be
accomplished by some other means than photosynthesis, yet it is composed
wholly of glucose--a natural photosynthetic product.

=Maltose= rarely occurs as such in plants, although its presence in the
cell-sap of leaves has sometimes been reported. It is produced in large
quantities by the hydrolysis of starch during the germination of barley and
other grains. This hydrolysis is brought about by the enzyme "diastase,"
which is present in the sprouting grain.

Maltose is easily soluble in water, and crystallizes in masses of slender
needles. It is a reducing sugar; readily forms a characteristic osazone; is
strongly dextrorotatory (specific rotatory power +137 deg.); and is readily
fermented by ordinary brewer's yeast, which contains both "maltase" (the
enzyme which hydrolyzes maltose to glucose) and "zymase" (the
alcohol-producing enzyme). When hydrolyzed, either by dilute acids or by
maltase, one molecule of maltose yields two molecules of glucose. Its
component hexoses are, therefore, the same as those of trehalose, a
non-reducing sugar, this difference in properties being due to the
difference in the point of linkage between the two glucose molecules, that
for maltose being such as to leave one of the aldehyde groups potentially
active, as shown in the following formula,

-------O------
| |
CH_{2}OH.CHOH.CH.CHOH.CHOH.CH
|
O
|
CHOH.CHOH.CHOH.CH.CHOH.CH_{2}
| |
----------O-----

=Isomaltose= is a synthetic sugar, obtained by Fischer, by condensing
two molecules of glucose. Its properties are quite similar to those of
maltose, but it yields a slightly different osazone and is not fermentable
by yeast. These differences are explained by the assumption that this
sugar is a glucose-[beta]-glucoside, while normal maltose is a
glucose-[alpha]-glucoside.

=Gentiobiose= is a disaccharide which results from the partial hydrolysis
of the trisaccharide _gentianose_ (see page 53). It is very similar in its
general p