=177. Influence of chloroplasts upon form and structure.= The beginning of all modifications produced by light stimuli must be sought in the chloroplast as the sensitized unit of the protoplasm. Hence, it seems a truism to say that the number and arrangement of the chloroplasts determine the form of the cell, the tissue, and the leaf, although it has not yet been possible to demonstrate this connection conclusively by means of experiment. In spite of the lack of experimental proof, this principle is by far the best guide through the subject of adaptations to light, and in the discussion that follows, it is the fundamental hypothesis upon which all others rest. The three propositions upon which this main hypothesis is grounded are: (1) that the number of chloroplasts increases with the intensity of the light; (2) that in shaded habitats chloroplasts arrange themselves so as to increase the surface for receiving light; (3) that chloroplasts in sunny habitats place themselves in such fashion as to decrease the surface, and consequently the transpiration due to light. In these, there can be little doubt concerning the facts of number and arrangement, since they have been repeatedly verified. The purpose of epistrophe and apostrophe, however, can not yet be stated with complete certainty. The stimulus of sunlight and of diffuse light is the same in one respect, namely, the chloroplasts respond by arranging themselves in rows or lines on the cell wall. The direct consequence of this is to polarize the cell, and its form changes from globoid to oblong. This effect is felt more or less equally by both palisade and sponge cells, but the disturbing influence of aeration has caused the polarity of the cells to be much less conspicuous in the sponge than in the palisade tissue. While the cells of both are typically polarized, however, they assume very different positions with reference to incident light. This position is directly dependent upon the arrangement of the plastids as determined by the light intensity. In consequence, palisade cells stand at right angles to the surface and parallel with the impinging rays; the sponge cells, conversely, are parallel with the epidermis and at right angles to the light ray. Some plants, especially monocotyledons, exhibit little or no polarity in the chlorenchym. As a result the leaf does not show a differentiation into sponge and palisade, and the leaves of sun and shade ecads are essentially alike in form and structure. The form of the leaf is largely determined by the chloroplasts acting through the cells that contain them. A preponderance of sponge tissue produces an extension of leaf in the direction determined by the arrangement of the plastids and the shape of the sponge cells, viz., at right angles to the light. Shade leaves are in consequence broader and thinner, and sometimes larger, than sun leaves of the same species. A preponderance