- 2072 i= 
parenchyma of turgid leaves is more densely packed, indivi- 
dual cells are turgid, and intercellular air spaces are smaller 
than those of non-turgid leaves. These findings are not cor- 
responding to current theories (WILLSTAETTER and STOLL 1918, 
SINCLAIR et al. 1973), which postulate larger differences in 
the near infrared reflectance. Further research needs ta be 
conducted on this problem. 
In viewing sugar beet plants, turgid as well as non-turgid, 
from the ground observer's eyes, it becomes evident that 
essential differences exist between both of them: (1) the plant 
canopy of non-turgid areas contains much less shadow spaces as 
compared to turgid plants; (2) a larger amount of the soil 
surface is visible; (3) a comparatively large petiole area is 
exhibited, and (4) the directional spectral reflectance char- 
acteristics of the leaves’ surfaces are changed as a result of 
their different angular positions. Thus, taking into consid- 
eration the structure of the plant canopy as a whole, whose 
significance for remote sensing applications has also been 
described by HILDEBRANDT (1976), a distinction betwees nema- 
tode infested and non-infested areas in a field by remote 
sensing techniques should be possible (SANWALD and HILDEBRANDT 
1978). This assumption proved to be correct: there are vi- 
sible changes in the colour tones of the sugar beet fields in 
the aerial photographs, which correspond exactly to the 
nematode-infested patches (see also Figure 4). 
Firstly, however, a dichotomous interpretation key for 
the classification of sugar beet fields from the infrared 
colour photographs was developped. This was done by the 
comparison of the photographs to the mapped areas, and 
also by comparing the photographs directly to the outdoor 
scene. Subsequently, another series of aerial photographs 
was classified for sugar beet fields by visual interpretation, 
and the results were checked on the mappings and in the fields. 
An accuracy of better than 98 per cent was achieved with 
respect to field numbers, and better than 99 per cent with 
respect to acreage. Optical density measurements and classi- 
fication experiments on the image analyzer confirmed the 
separatibility between sugar beets and the somewhat similar