PHOTOHARMOSE 143 
absorption of light in the palisade reduces the intensity to such a degree 
that the cells of the lower half of the leaf are in diffuse light, and are in 
consequence modified to form sponge tissue. The sponge tissue of the 
diphotic leaf is just as clearly an adaptation to diffuse light as it is in 
those plants where the whole chlorenchym is in the shade of other plants 
or of a covering of hairs. As is indicated later, all these relations permit of 
ready confirmation by experiment, either by changing the position of the 
leaf or by modifying the intensity or direction of the light. 
The preceding discussion 
makes it fairly clear that 
sponge tissue is developed 
primarily to increase the light- 
absorbing surface. Because 
of its direct connection with 
photosynthesis, the sponge tis- 
ste is the especial organ of 
aeration, also, and since it 
shows a high development of 
air spaces for this purpose, it 
is inevitably concerned in 
transpiration. It seems to be 
partly a coincidence, however, 
that the sponge is found next 
to the lower surface upon 
which the stomata are most 
numerous. This is indicated 
by artificial ecads of Ranun- 
culus  sceleratus, in which 
sponge tisstie is unusually de- 2 
veloped, although the stomata Fig. 43. A plastic species, Wertensia poly- 
are much more numerous upon phylla, showing the effect of water upon the 
the upper surface, Palisade ae 1, chresard 25%; 2, chresard 129. 
tissue is apparently developed 
primarily as a protection against water loss, particularly that due to the 
absorption of light by the chloroplast. The small size of the intercellular 
passages between palisade cells likewise aids in decreasing transpiration. 
The fact that leaves with much palisade tissue transpire twice a much as 
shade leaves is hardly an objection to this view, as Hesselmann 
(1. ¢., 442) would think. It is readily explained by the intense photosyn- 
thesis of sun plants, which makes necessary an increase, usually a doubling, 
in the number of stomata, in consequence of which the transpiration is 
increased. 
