138 THE PLANT 



1. bright yellow or leather yellow (no starch in the chlorenchym) 



2. blackish (very little starch in the chlorenchym) 



3. dull black (starch abundant) 



4. coal black (starch very abundant) 



5. black, with metallic luster (maximum starch-content) 



ADAPTATION 



177. Influence of chloroplasts upon form and structure. The begin 

 ning 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 de- 

 termine 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 fundamential hypothesis upon which all 

 others rest. The three propositions upon which this main hypothesis is 

 grounded are : ( i ) that the number of chloroplasts increases with the in- 

 tensity of the light; (2) that in shaded habitats chloroplasts arrange them- 

 selves 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 difi^use 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 



