CHLOROPLASTS 253 



of the chlorophyll in which its absorption comes nearest to that of 

 the living plastids, which would permit us to predict how it will 

 behave in the chloroplast. 



According to Kundt's law, the position of the absorption bands 

 is governed by the refractive index of the solvent, in the sense that 

 the hio-her the refractive index, the more will the bands shift towards 

 the long-wave region. This, however, applies only to a limited extent 

 to chlorophyll, viz., only in so far as solvents of equal chemical value 

 are compared. Thus Wakkie finds four different series of substances 

 to which Kundt's law applies ; they are : i . purely lipidic liquids like 

 heptane, carbon tetrachloride, benzene; 2. ethyl ether and ketones; 

 3. alcohols; 4. water, glycerol. In the lipidic solvents the red ab- 

 sorption band is shifted farthest from its natural position towards 

 yellowy in the ketones somewhat less so (e.g., acetone 6640 A) and in 

 the alcohols sdll less so (ethyl alcohol 6665 A, benzyl alcohol 6720 A). 

 Hence, the more hydrophilic the solvents, the closer is the approach 

 to natural conditions in the leaf. The position of the absorption bands 

 cannot, therefore, be improved by adding lipids (Na oleate) to 

 alcoholic solutions; on the contrary, it is worsened by 20 A. Solutions 

 in water most nearly approximate the natural green of leaves (6720 A); 

 despite the fact that the chlorophyll is dissolved coUoidally, and not 

 molecularly, in this lipophobic solvent, the effect of the increased 

 hydrophilic bias is to strengthen the resemblance to the conditions 

 existing in the living chloroplast. Since it does not seem possible to 

 find solvents in which chlorophyll displays the same absorption 

 maximum as in the leaf, it must be assumed that the chlorophyll is 

 not dissolved, but chemically combined in the chloroplast. 



Birefringence. Very important criteria are supplied by the bire- 

 fringence of the chloroplasts and phaeoplasts. It was discovered by 

 ScARTH (1924) and was found to be widespread by Kuster (1933, 

 1935b), Menke (1934a, b, 1943), Ullrich (1936a) and Weber (1937)- 

 The Weber school rightly ascribes the optical anisotropy to the lipidic 

 substances, which can be made to emigrate; they then produce 

 striking birefringent myelin forms (Weber, 1933; Menke, 1934a). 



KiJSTER (1933, 1937) and Menke (1934b) discovered the lamelli- 

 form chloroplasts of Mougeotia, Mesocarpus, Spirotaenia, Spirog^fra and 

 other algae to be clearly birefringent in profile and in cross-section, 

 viz., negative with reference to the thickness of the plastids; the top 



