13.6 THE PLANT 



to the impossibility of distinguishing between the water loss due to light 

 and that caused by humidity and other factors. If it were possible to de- 

 termine the amount of chlorophyll or glucose produced, these could be used 

 as satisfactory measures of response. As it is, they can only be determined 

 approximately by counting the chloroplasts or starch grains. The arrange- 

 ment of the chloroplasts can not furnish the measure sought, since it does 

 not lend itself to quantitative methods, and since the relation to light in- 

 tensity is too inconstant. Hesselmann (/. c, 400) has determined the 

 amount of carbon dioxide respired, by means of a eudiometer, and has based 

 comparisons of sun and shade plants upon the results. As he points out, 

 however, light has no direct connection with respiration. Although the 

 latter increases necessarily with increased nutrition, the relation between 

 them is so obscure, and so far from exact, that the amount of respiration 

 can in no wise serve as a measure of the response to light. As a result of 

 the foregoing, it is clear that no functional response is able to furnish a 

 satisfactory measure of adjustment to light, though one or two have per- 

 haps sufficient value to warrant their use. Indeed, structural adaptations 

 otYer a much better basis for the quantitative determination of the effects 

 of light stimuli, as will be shown later. 



In attempting to use the number of chloroplasts or starch grains as a 

 measure of response, the study should be confined to the sun and shade 

 forms of the same species, or, in some cases, to the forms of closely related 

 species. The margin of error is so great and the connection with light 

 sufficiently remote that comparisons between unrelated forms or species 

 are almost wholly without value. It has already been stated that starch is 

 merely the surplus carbohydrate not removed by translocation ; the amount 

 of starch, even if accurately determined, can furnish no real clue to the 

 amount of glucose manufactured. In like manner, the number of chloro- 

 plasts can furnish little more than an approximation of the amount of 

 chlorophyll, unless size and color are taken into account. In sun and shade 

 ecads of the same species, the general functional relations are essentially 

 the same, and whatever differences appear may properly be ascribed to 

 different light intensities for the two habitats. The actual counting of 

 chloroplasts and starch grains is a simple task. Pieces of the leaves of the 

 two or more forms to be compared are killed and imbedded in paraffin in 

 the usual way. To save time, the staining is done in toto. Methyl green is 

 used for the chloroplasts and a strong solution of iodine for the starch 

 grains. When counts are to be made of both, the leaves are first treated 

 with iodin and then stained with the methyl green. The thickness of the 

 microtome' sections should be less than that of the palisade cells in order 

 that the chloroplasts may appear in profile, thus facilitating the counting. 

 The count is made for a segment 100 /u, in width across the entire leaf. 



