PHOTOHARMOSE 133 
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 
have 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 celis 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 aposirophe, 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 splants is such a 
