The absorption of the light stimulus by the green plastid results, under normal conditions, in the immediate production of carbohydrates, which in the vast majority of cases soon become visible as grains of starch. The appearance of starch in the chloroplasts of flowering plants is such a regular response to the action of light that it is regarded as the normal indication of photosynthetic activity. The mere presence of chlorophyll is not an indication of the latter, since chlorophyll sometimes persists in light too diffuse for photosynthesis. The amount of starch formed is directly connected with the light intensity, and in consequence it affords a basis for the quantitative estimation of the response to light. Two responses to light stimuli have a direct effect upon the amount of transpiration. Of the light energy absorbed by the chloroplast, only 2.5 per cent is used in photosynthesis, while 95–98 per cent is converted into heat, and brings about marked increase in transpiration. Furthermore, in normal turgid plants, the direct action of light, as is well known, opens the stomata in the morning and closes them at night. [Illustration: Fig. 38. Ecads of _Allionia linearis_, showing position of chloroplasts. The palisade shows apostrophe, the sponge epistrophe: 1, sun leaf (chresard, 2–5%, light, 1); 2, shade leaf (chresard, 11%; light, .012); 3, shade leaf (chresard, 11%; light, .003). × 250.] =175. Aeration and translocation.= The movements of gases and of solutions through the tissues of the leaf are intimately connected with photosynthesis, and hence with responses to light stimuli. Aeration depends primarily upon the periodic opening of the stomata, for, while the carbon dioxide and oxygen of the air are able to pass through epidermal walls not highly cutinized, the amount obtainable in this manner is altogether inadequate, if not negligible. The development of sponge tissue or aerenchym is intimately connected with the stomata. The position and amount of aerenchym and the relative extent of sponge cells and air-spaces are in part determined by the number and position of the breathing pores. The disposition of air spaces has much to do with the arrangement of chloroplasts in both palisade and sponge tissues. Starch formation is also dependent upon the presence of air spaces, but, contrary to what would be expected, it seems to be independent of their size, since sun leaves, which assimilate much more actively than shade leaves, have the smallest air spaces. From this fact, it appears that the rapidity of aeration depends very largely upon the rapidity with which the gases are used. Translocation likewise affects the arrangement of the chloroplasts and the formation of starch. According to Haberlandt, it also plays the principal part in determining the form and arrangement of the palisade cells. Chloroplasts are regularly absent at those points of contact where the transfer of materials is made from