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 tissue 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 _Ranunculus sceleratus_, in which sponge tissue is unusually developed, although the stomata are much more numerous upon the upper surface. Palisade 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 as much as shade leaves is hardly an objection to this view, as Hesselmann (_l. c._, 442) would think. It is readily explained by the intense photosynthesis of sun plants, which makes necessary an increase, usually a doubling, in the number of stomata, in consequence of which the transpiration is increased. [Illustration: Fig. 44. A stable species, _Erigeron speciosus_: 1, sun leaf; 2, shade leaf (light .03). × 130.] [Illustration: Fig. 45. Spongophyll of _Gyrostachys stricta_ (light 1). × 130.]