ECOLOGY OF WOODLAND PLANTS. 367 
fications usually attributed, for example, to the direct influence 
of light, mechanical forces (as strain), &c., he would consider 
rather as pathological states due to insufficient nourishment, 
lowered transpiration, and the like. 
Again, the results obtained by the examination of the tissues 
of plants exposed to sun or shade are sometimes contradictory. 
It is generally admitted that what are known as *'sun-leaves " 
are developed in situations exposed to intense sunlight, as on 
sand-dunes, moorlands, Alpine regions, as well as under less 
rigorous conditions. 
Leist (56), however, in examining the leaves of Alpine plants, 
came to the conclusion that they showed mesophytic rather than 
xerophytie, shade rather than sun, structures, and gave as an 
explanation the considerable period during which the mountains 
are enveloped by clouds. 
Wagner (97), on the other hand, came to the opposite 
conclusion, and found that the “sun” type and xerophytie 
structures became more pronounced with increased altitude and 
the accompanying xerophytie conditions. This accords with the 
observations of Bonnier (7) aud others, and also that transpiration 
and assimilation are augmented at high altitudes. 
Wiesner's observations on transpiration (104) show that under 
the same conditions shade-leaves transpire more than sun-leaves. 
Bergen (5), however, in his study of the evergreens of the 
Mediterranean region, concluded that sun-leaves transpired 
more than shade-leaves under the same conditions. 
The investigations of Ball (3) as to the value of stresses in 
the development of mechanical tissues has considerably modified 
our views on tbis subject ; from his results we are no longer able 
to give such an important place to the effects of the stimulus of 
stresses as we were led to do by the earlier experiments of 
Hegler (46). 
These instances will be sufficient to show that so-called adaptive 
structures will require in the future re-investigation, and that 
we are scarcely justified, in spite of the considerable work that 
has already been accomplished, in attributing dogmatically these 
modifications to particular factors. We may record the modi- 
fications and observe the conditions under which they are 
produced as far as we can discern them, but prolonged study, 
not only in the laboratory, but also in the field, will be necessary 
before satisfactory conclusions can be drawn. . 
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