PHOTOHARMOSE 1 33 



plastids. The position of air-spaces as reservoirs of carbon dioxide and the 

 movement of crude and elaborated materials from cell to cell frequently 

 have much to do with this problem. Finally, it must be constantly kept in 

 mind that the chloroplasts lie in the cytoplasm, which is in constant contact 

 with a cell wall. Hence, any force that affects the shape of the cell will 

 hav a corresponding influence upon the position of the chloroplasts. 

 When it is considered that in many leaves these four factors play some part 

 in determining the arrangement of the plastids, it is not difficult to under- 

 stand that anomalies frequently appear. 



It may be laid down as a general principle that chloroplasts tend to place 

 themselves at right angles to rays of diffuse light and parallel to rays of 

 sunlight. This statement is borne out by an examination of the leaves of 

 typical sun and shade species, or of sun and shade forms of the same 

 species. Cells which receive diffuse light, i. e., sponge cells, normally have 

 their rows of plastids parallel with the leaf surface, while those in full 

 sunlight place the rows at right angles to the surface. This disposition at 

 once suggests the generally accepted view that chloroplasts in diffuse light 

 are placed in such a way as to receive all the light possible, while those in 

 sunlight are so arranged as to be protected from the intense illumination. 

 Many facts support this statement with respect to shade leaves, but the need 

 of protection in the sun leaf is not clearly indicated. The regular occurrence 

 of normal chloroplasts in the guard cells seems conclusive proof that full 

 sunlight is not injurious to them. Although the upper wall of the outer 

 row of palisade cells is usually free from chloroplasts, yet it is not at all un- 

 common to find it covered by them. These two conditions are often found 

 in cells side by side, indicating that the difference is due to the presence 

 of carbon dioxide and not to light. In certain species of monocotyledons, 

 the arrangement of the chloroplasts is the same in both halves of the leaf, 

 and there is no difference between the sun and shade leaves of the same 

 species. The experimental results obtained with concentrated sunlight, 

 though otherwise conflicting, seem to show conclusively that full sunlight 

 does not injure the chloroplasts of sun plants, and that the position of 

 plastids in palisade cells is not for the purpose of protection. This arrange- 

 ment, which is known as apostrophe, is furthermore often found in shade 

 forms of heliophytes. In typical shade species, and in submerged plants, 

 the disposition of plastids on the wall parallel with the leaf surface, viz., 

 epistrophe, is more regular, but even here there are numerous exceptions 

 to the rule. 



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 



