THE GRANA 361 



be seen on photographs showing them from the side (e. g., Fig. 39c). 

 The large diameter of the grana is from 0.3 to 2 /x according to Heitz 

 (1936) and Baas-Becking and Hanson (1937). They are larger in shade 

 plants than in sun plants; in a given leaf, their dimensions are larger in 

 the spongy parenchyma than in the palisade tissue. The drawing in 

 figure 39d shows the variations in the sizes of the grana in various plants. 

 On the far right, in the specimen with the few large grana, one is shown 

 in the process of division. (According to Heitz, grana generally propa- 

 gate by division.) 



Heitz' and Doutreligne's photographs show that the number of grana 

 in a chloroplast can vary from ten to a hundred. Hanson (1939) has 

 counted grana in 50 chloroplasts of Hormidium flaccidum; the average 

 was 26 grana per chloroplast. 



According to Neish (1939), when chloroplasts are allowed to swell in 

 distilled water, they disintegrate, and the chlorophyll-bearing grana 

 (discs or spheres, with a size of about one-sixth of the whole chloroplast) 

 are set free; these swell very slowly and do not rupture for weeks. Heitz 

 (1936), Neish (1939), and Mommaerts (1938) asserted that chloroplast 

 preparations made by grinding the leaves in pure water and fractionating 

 the triturate, contain free grana rather than whole chloroplasts or their 

 fragments; but this view was opposed by Menke (1940) (c/. page 370). 



What is the difference in composition between grana and stroma? 

 Menke (1938) suggested that it is quantitative rather than qualitative — 

 that the grana are vaguel}^ outlined regions in which the concentration 

 of certain components (e. g., pigments) is higher than in the rest of the 

 chloroplast (c/. page 363). It is known (cf. page 371 et seq.) that proteins 

 form about 50% of the chloroplast matter, and lipoids (ether-soluble 

 compounds) about 30%. It is thus natural to assume that the grana 

 differ from the stroma in the relative proportion of these two types of 

 materials. Wieler (1936) observed that grana can be dissolved in alcohol, 

 leaving cavities in the stroma; he therefore considered the grana as the 

 more lipophilic part of the chloroplast. Weier (1936) on the other hand, 

 thought, that only the pigments are extracted by alcohol, leaving behind 

 discolored grana (rather than cavities); he suggested that the grana 

 consist mainly of hydrophilic (proteinaceous) material. 



An accumulation of lipoids in the grana, assumed by Wieler, is 

 supported by staining experiments (according to Strugger 1936, and 

 Wieler 1936, the grana are preferentially stained by lipophilic dyestuffs, 

 e. g., rhodamine B, and Sudan red III) and, according to Frey-Wyssling 

 (1938), by the observation (attributed by him to Metzner) that grana 

 melt upon heating. This transition is accompanied by a sudden increase 

 in the intensity of fluorescence, which can be noticed under the fluores- 

 cence microscope (cf. Vol. II, Chapter 24). 



