PHOTOHARMOSE 129 
stoppage of the stemata by water. Stomata, except as vestiges, are found 
only on the upper surface, and the palisade tissue is much less developed 
than the sponge, which is uniformly characterized by large air-chambers. 
The stems are elongated, the aerating system is enormously developed, and 
the supportive tissues are reduced. In the Lemmnaceae, the leaf and the 
stem are represented by a mere frond or thallus, and the roots are in the 
process of disappearance, e. g., Spirodela has several, Lemna one, and 
W oiffia none. 
3. The submerged type. Both stem and root have been greatly reduced 
in submerged plants, owing to the generalization of absorption and the 
density of the water. The leaves are greatly reduced in size and thickness, 
chiefly, it would seem, for the purpose of insuring readier aeration and great 
illumination. The leaf may be ribbon-like, linear, cylindrical, or finely 
dissected. Stomata are sometimes present, but they are functionless and 
vestigial. A distinction into palisade and sponge tissues, when present, 
must also be regarded as a vestige; the chlorenchym is essentially that of a 
shade leaf. The air chambers are much reduced, and sometimes lacking; 
they function doubtless as reservoirs for air obtained from the water. 
PHOTOHARMOSE 
ADJUSTMENT 
172. Light as a stimulus. In nature, light stimuli are determined by 
intensity and not by quality. A single exception is afforded by those aquatic 
habitats where the depth of water is great, and in consequence of which 
certain rays disappear by absorption more quickly than others. In forests 
and thickets, where the leaves transmit only the green and yellow rays, it 
would appear that the light which reaches the herbaceous layers is deficient 
in red and violet rays. The amount of light transmitted by an ordinary 
sun leat is so small, however, that it has no appreciable effect upon the 
quality of the light beneath the facies, which is diffuse white light that has 
passed between the leaves. Indeed, it is only in the densest forests that 
distinct sunflecks do not appear. Coniferous forests, with a light value 
less than .oo5, which suffices only for mosses, lichens, and a few flowering 
plants, show frequent sunflecks. This is convincing evidence that the light 
of such habitats is normal in quality. It warrants the conclusion that in 
all habitats with an intensity capable of supporting vascular plants the light, 
no matter how diffuse, is white light. The direction of the light ray is of 
slight importance in the field, apart from the difference in intensity which 
may result from it. In habitats with diffuse light, the latter comes normally 
and constantly from above. Likewise, in sunny situations, direction can 
