Fig. 4. The amount of carbon incorporated in organic compounds
is a measure of the amount of organic matter produced. Within
any given area, cultivated vegetation is less efficient than forest
vegetation, but more efficient than for grassland vegetation.
Increasing the productivity of the vast areas of desert on the
surface of the globe may be our greatest scientific challenge.
[From E.S. Deevey, The Human Population. Copyright © 1970
by Scientific American, Inc. All rights reserved.]