132. THE PLANT 
chloroplasts in the epidermal cells. Such cases merely serve to confirm the 
view that the perception of light stimuli is localized in the chloroplast. In 
conformity with this view, the initial response to such stimuli must be sought 
in the chloroplast, and the explanation of all adaptations due to light must 
be found in the adjustment shown by the chloroplasts. 
174. Response of the chloroplast. The fundamental response of a 
plastid to light is the manufacture of chlcrophyll. In the presence of carbon 
dioxide and water, leucoplasts invariably make chlorophyll, and chloroplasts 
replace that lost by decomposition, in response to the stimulus exerted by 
light. The latter is normally the efficient factor, since water is always 
present in the living plant, and carbon dioxide absent only locally at most. 
Sun plants which possess a distinct cuticle, however, produce leucoplasts, 
not chloroplasts, in the epidermal cells, although these are as strongly 
illuminated as the guard cells, which contain numerous chloroplasts. This 
is evidently explained by the lack of carbon dioxide in the epidermis. This 
gas is practically unable to penetrate the compact cuticle, at least in the 
small quantity present in the air. The supply obtained through the 
stomata is first levied upon by the guard cells and then by the cells of the 
chlorenchym, with the result that the carbon dioxide is all used before it 
can reach the epidermal cells. This view is also supported by the presence 
of chloroplasts along the sides and lower wall of palisade cells, where 
there is normally a narrow air-passage, and their absence along the upper 
wall when this is closely pressed against the epidermis, as is usually the 
case. Furthermore, the leaves of some mesophytes when grown in the 
sun develop a cuticle and contain leucoplasts. Under glass and in the 
humid air of the greenhouse, the same plants develop epidermal chloroplasts 
but no cuticle. This is in entire harmony with the well-known fact that 
shade plants and submerged plants often possess chloroplasts in the 
epidermis. Although growing in different media, their leaves agree in 
the absence of a,cuticle, and consequent absorption of gases through the 
epidermis. The size, shape, number, and position of the chloroplasts are 
largely determined by light, though a number of factors enter in. No 
accurate studies of changes in size and shape have yet been made, though 
casual measurements have indicated that the chloroplasts in the shade form 
of certain species are nearly hemispherical, while those of the sun form are 
plane. In the same plants, the number of chloroplasts is strikingly smaller 
in the shade form, but exact comparisons are yet to be made. The position 
and movement of chloroplasts have been the subject of repeated study, but 
the factors which control them are still to be conclusively indicated. Light 
is clearly the principal cause, although there are many cases where a marked 
change in the light intensity fails to call forth any readjustment of the 
