PHOTOHARMOSE 129 



stoppage of the stomata by water. Stomata, except as vestiges, are found 

 only on the upper surface, and the pahsade 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 Lemnaceae, 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 

 WoMa 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 leaf 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 .005, 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 



