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 ail 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 chlorophyll. 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 



