mpounds makes their study difficult
and their functions uncertain.

The term =cuto-celluloses= is applied to the group of substances, including
suberin and cutin, which constitute waterproof cell-walls. These were
formerly supposed to consist of true cellulose impregnated with fatty or
wax-like materials. Recent investigations seem to indicate, however, that
there is really no cellulose nucleus in such walls as these, but that they
are compound glyceryl esters resembling the true fats (see chapter X) in
composition. If this view should finally be established as a fact, this
sub-group of supposed compound celluloses should be dropped from
consideration as such.

PHYSIOLOGICAL USE OF CELLULOSES

There seems to be no question that the sole use of celluloses is to serve
as structure-building materials. They are undoubtedly elaborated from the
carbohydrates as the cell grows. In only rare cases, however, is there any
evidence that they can be reconverted into carbohydrates to serve as food
material. Certain bacteria can make use of cellulose as food, and secrete
an enzyme, cytase, which aids in the hydrolysis of cellulose to sugars for
this purpose. But this enzyme seems rarely, if at all, to be present in the
tissues of higher plants. It has been reported that some cellulose is
hydrolyzed during the malting of barley, indicating that this might have
some food use for the growing seedling; but this observation has not been
confirmed and later investigations seem to throw doubt upon its accuracy.

Bacteria of decay also act upon cellulose materials, converting them
chiefly into gaseous products; but this seems to be a provision of nature
for the destruction of the cell-wall material of dead plants, rather than
an arrangement for the constructive use of it as food for the bacterium.
When fibrous plant residues decay in the soil, the cellulose compounds are
first converted into a series of complex organic acids, known as "humins,"
which undoubtedly have a significant effect upon the chemical and physical
properties of the soil, but these have little interest or significance in a
study of the chemistry of plant growth.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2, Gummisubstanzen,
Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band
8--1 Ergaenzungsband (same title as Band 2)--," 507 pages, Berlin, 1914.

SCHWALBE, C. G.--"Di