362 CHLOROPLASTS AND CHROMOPLASTS CHAP. 14 



Older investigators (Meyer 1883; Schimper 1885) were not certain 

 whether all pigments are segregated in the grana or not. The grana 

 undoubtedly were darker than the stroma, but the latter did not appear 

 quite colorless. Doutreligne (1935) made photographs in monochromatic 

 light, in the hope that, if chlorophyll were concentrated in the grana, 

 the contrast would be strong in red and blue, and absent in green and 

 infrared. There was (as expected) almost no contrast in infrared light, 

 but the pictures taken in green light were not very different from those 

 in red and blue light. However, this may be due to the fact that chloro- 

 phyll absorbs much more strongly in the green than in the infrared {cf. 

 Vol. II, Chapter 21). 



Heitz (1932) found that the stroma of many chloroplasts is entirely 

 colorless. In other species, the picture was less clear — probably because 

 of light scattering, rather than because of an actual coloration of the 

 stroma. 



The distribution of chlorophyll between stroma and grana also can 

 be studied by means of a fluorescence microscope. Lloyd (1923) asserted 

 that only the stroma fluoresces, while the grana remain dark. This 

 result has not been confirmed by Metzner (1937), who asserted that on 

 the contrary, the grana alone are fluorescent, and claimed that this 

 difference can be used to detect the grana in specimens which do not 

 show them clearly by transmitted light. The settlement of this contro- 

 versy is desirable, since it would help to understand the state and distri- 

 bution of chlorophyll in the chloroplasts (cf. Vol. II, Chapter 24). Lloyd's 

 observation could be quoted in support of the concept of Seybold and 

 Egle that chlorophyll exists in plants in two forms — in a concentrated, 

 nonfluorescent form in the grana, and in a diluted, strongly fluorescent 

 form in the stroma (cf. page 392) ; while Metzner's result supports the 

 alternative (and more plausible) hypothesis that all chlorophyll is 

 contained in the grana and is in a w^eakly fluorescent form. 



3. The Lamina 



It was stated above that, according to Heitz (1936), the grana usually 

 are arranged in layers. Menke (1938) has concluded from the bire- 

 fringence of the chloroplasts (cf. pages 365 et seq.) that these organs have 

 a laminar structure; and Menke and Koydl (1939) observed the disinte- 

 gration of microtome slices of Anthoceros chloroplasts into stacks of 

 laminae, pushed apart — particularly in the middle of the chloroplasts — 

 by the pressure of accumulated assimilates. They counted from 20 to 

 40 of these laminae in each slice; and since the assimilate-free chloroplasts 

 were only 1-2 ^t thick, the thickness of a single lamina must have been 

 of the order of 0.05 ^l (that is, below the limit of dissolution of an ordinary 

 microscope). That they could be seen at all must have been due to the 



