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TROPICAL VEGETATION AND CROPS, MILLER 131
most cases probably a reflection of the drainage or the underlying rock structure. In the
very different conditions of the peat-swamp forests of Sarawak small pit-like gaps and
others much larger and with radiating arms are conspicuous on aerial photographs.
Anderson of the Sarawak Forest Department has, since noting them on the photographs,
been able to explain these features as due respectively to lightning strikes and windblows
from cyclonic storms. He has also spotted other areas of recent and past devastation in
these Shorea albida forests which, from scanty native information, he deduces to be the
result of trees being killed by a defoliating caterpillar, although no specimen of the in-
sect itself has yet been obtained. In still another category are the varied but character-
istic patterns formed by fires in open grassland. In all these cases the aerial photographs
have indicated features either too diffuse on the ground or too embedded in dense jungle
for their shape or pattern to be apparent to the worker in the field.
The variety of crops cultivated in the tropics is almost as great as that of the natural
vegetation, ranging from tall coco-nut palms to low groundnut plants. There are cer-
tainly more tree and bush crops grown in the humid tropics than in any other part of
the world, and this widens the scope for identification on aerial photographs. The lay-
out of the crops is, however, often a greater aid to recognition than the shape or even
the size of the plants concerned. In most regions only a few crops are cultivated in large,
regular plantations, and with sufficient background information these can generally be
identified even on small-scale photographs. Plantations of such crops as rubber, coco-nut,
tea, sisal and sugar-cane all have distinct characters in any one region: these may be as
much a reflection of the type of site on which the crop is grown, the method of culti-
vation and the means of harvesting as of the nature of the plants themselves. Sugar cane
plantations, in fact, will contain fields at many different stages of growth and wil! show
such a variety of tones that it may be difficult at first to believe that they are all the
same crop. Associated features such as drying sheds for tea, racks for sisal fibre and the
earth-works and polders of swamp rice are, of course, additional clues in the recognition
of these more elaborate agricultural enterprises.
Many tropical erops are grown in a much more dispersed manner, and the photo-
interpreter is then confronted with more difficult problems. Some perennial plants are
encouraged or allowed to develop in a semi-wild state. Oil-palms are an obvious example
of this and of a crop which may be screened from the camera by taller vegetation. Cacao,
too, is generally planted in West Africa beneath the shade of natural forest trees. Nut-
megs and cloves, although they are trees of good size, usually occur in a haphazard man-
ner and have little to distinguish them from other wild or cultivated species. Even coco-
nut palms, which are so distinctive in the open or in plantations, can be lost from view
to the photo-interpreter when they are surrounded by secondary growth. With crops of
this type it may be possible to recognise or deduce their presence on aerial photographs,
but it is well-nigh impossible to define the exact area which they occupy.
Most of the subsistence crops of the tropics and many of the cash Crops are grown
on small peasant holdings which are irregular in shape and size and may contain a
mixture of perennial and annual crops. In such a mosaic the interpreter needs high
quality photographs and a good understanding of local agricultural practice if he is to
obtain the maximum amount of information from them. Single plants of bananas and
the larger bush-crops grown in the open are visible at most scales of photography, but
for cereals and ground-crops the evidence of row spacings or the pattern of planting
mounds may have to be used to infer the crops growing on them. Cassava, for instance,
has been deduced from the different tone of patches of subsoil thrown up when the
roots were dug out. Mixed cropping is another common feature of peasant agriculture in
the tropics, which causes trouble to the photo-interpreter as well as to the collector of
agricultural statistics. Interspersed with the crops there will be patches of natural fal-
low, which is the principal means used by the peasant farmer in the tropics for the res-
