PHOTOHARMOSE 



H3 



absorption of light in the paHsade 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 w'ith 

 photosynthesis, the sponge tis- 

 sue 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 tissue is unusually de- 

 veloped, 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 a much as 

 shade leaves is hardly an objection to this view, as Hesselmann 

 (/. c, 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. 



Fig. 43. A plastic species, Mertensia poly- 

 phylla, showing the effect of water upon the 

 sponge: 1, chresard 25^; 2, chresard Vl%. 

 X130. 



