PHOTOHARMOSE 137 



Two segments in different parts of the section are counted, and the result 

 multipHed by five to give the number for a segment i milHmeter in width. 

 Although sun and shade leaves regularly differ in size and thickness, no 

 correction is necessary for these. Size and thickness stand in reciprocal 

 relation to each other in ecads, and thickness is largely an expression of 

 the absorption of light, and hence of its intensity. In the gravel, forest, and 

 thicket ecads of Galium boreale, counts of the chloroplasts gave the follow- 

 ing results. The gravel form (light i) showed 3,500 plastids in the i-mm. 

 segment, the forest form (light .03) possessed 1,350, and the thicket form 

 (light .002), 1,000. In these no attention was paid to the size and form 

 of the plastids in the different leaves, since the differences were inappre- 

 ciable. When this is not the case, both factors should be taken into account. 

 Starch grains are counted in exactly the same way. Indeed, if care is taken 

 to collect leaves of forms to be compared, at approximately the same time 

 on sunshiny days, a count of the chloroplasts is equivalent to a count of the 

 starch grains in the vast majority of cases. Measurements of the size of 

 starch grains can be made with accuracy only when the leaves are killed in 

 the field at the same time, preferably in the afternoon. Counts of chloro- 

 plasts alone can be used as measures of response in plants that produce 

 sugar or oil, while either chloroplasts or starch grains or both may be 

 made the basis in starch-forming leaves. 



Hesselmann (/. c, 379) has em.ployed Sachs's iodine test as a measure 

 of photosynthesis. This has the advantage of permitting macroscopic ex- 

 amination, but the comparison of the stained leaves can give only a very 

 general idea of the relative photosynthetic activity of two or more ecads. 

 The iodine test is made as follows '} fresh leaves are placed for a few 

 minutes in boiling water, and then in 95 per cent alcohol for 2-5 minutes, 

 in order to remove the chlorophyll and other soluble substances. The leaves 

 are placed in the iodine solution for 5^2-3 hours, or until no further change 

 in color takes place. The strength of the solution is not clearly indicated 

 by Sachs, who says : 'T used an alcoholic solution of iodin v/hich is best 

 made by dissolving a large quantity of iodin in strong alcohol and adding 

 to this sufficient distilled water to give the liquid the color of dark beer." 

 This solution may be approximated by dissolving 1/3 gram of iodin in 100 

 grams of 30 per cent alcohol. The stained leaves are put in a white porce- 

 lain dish filled with distilled water, and the dish placed in the strong 

 diffuse light of a window. The colored leaf stands out sharply against the 

 porcelain, and the degree of coloration, and hence of starch content, is 

 determined by the following table : 



' Sachs, J. Ein Beitrag zur Kenntniss der Ernahrungsthatigkeit der Blatter. 

 Gesammelte Abhandlungen iiber Pflanzenphysiologie. 1:355. 1892. 



