ADJUSTMENT TO LIGHT 87 



owe the differentiation of the chlorenchym into paUsade and 

 sponge tissues to this fact. The upper part of the leaf receives 

 full sunlight, and the plastids place themselves in line with the 

 light rays. The lower portion receives only the light that is not 

 absorbed by the upper. It is just as truly shaded as a leaf growing 

 in a forest, and the chloroplasts spread out in such fashion as 

 to receive as much light as possible. This arrangement is the 

 rule, but it is not absolute, owing to local modifications due to the 

 position of air passages, and the necessity of diffusion from cell 

 to cell. The position of the plastids influences the form of the 

 cells, so that, while both palisade and sponge cells are more or 

 less elongated, the palisade cells are placed at right angles to the 

 leaf surface, and the sponge cells are parallel to it. This relation 

 is doubtless connected with the fact that sun leaves are generally 

 narrower and thicker in comparison with shade leaves, which are 

 thinner and broader. The influence of light upon the form and 

 structure of the leaf is considered under Adaptation. 



Experiment 28. Arrangement of chloroplasts. Fix one of a 5''oung 

 pair of leaves of a sunflower with the lower surface uppermost and the 

 other in an erect position. After they have become fully grown, cut 

 cross-sections of each, and compare with the cross-section of a leaf 

 in the normal position. Make drawings of changes in the number and 

 arrangement of the chloroplasts, as well as in the shape of the cells. 



108. Movement of stems and leaves. Through the long- 

 continued action of light, the stems and leaves of plants have 

 acquired certain habits of growth or position. A change in the 

 normal relation results in an attempt upon the part of the ])lant 

 to assume a corresponding position. Stems usually place them- 

 selves in line with the light rays, while leaves tend to stand at 

 right angles to them. If the direction of the light is changal, either 

 by shutting it off from all sides but one, or by changing the posi- 

 tion of the plant, the stem tries to bend in such a way as to keep 

 the same position with relation to the light. Leaves when in- 

 verted or otherwise changed from the normal position twist and 

 turn in an endeavor to resume it. In somewhat similar fashion, 

 roots, which are organs habituated to the absence of light, in 

 some plants at least bend away from the light, if they are free to 

 (U) so. Finally, light produces certain continuous periodical 

 changes in the position of the leaf. 



