3 CHLOROPLASTS 245 



we are compelled to assume that the grana are often submicroscopic 

 and only become visible by coarsening. Experience of nuclear struc- 

 tures would seem to imply that, again, it is not a matter of artefacts 

 in this case, but rather of pre-formed structures which, lying below 

 microscopic resolving power, or exhibiting no optically demonstrable 

 phase boundaries, have become visible. The second alternative at the 

 same time shows why the chloroplasts appear to be optically empty 

 in the ultramicroscope (Guilliermond, 1930). 



Heitz declares that the grana vary in size from 0.5 to 2 /< and that 

 the size is specific to the species. As against this, the granules in light 

 plants are always found to be smaller than in shade plants ; accordingly, 

 the granular size increases from the upper side of the leaf (palisades) 

 towards the underneath (spongy tissue). The grana are especially large 

 and distinct in the chloroplasts of the green fruit of Polygonatum 

 (Menke, 1934a, who, however, calls them artificial products; Weber, 

 1936). 



The evidence that the grana are not globules, but platelets, is im- 

 portant (Heitz, 1936b). In the side view of the flat discs of chloro- 

 plasts they look like dense streaks (cf. Fig. 130b, p. 25 5). The Heitz 

 microphotographs reveal no localization of the grana in the periphery; 

 this conflicts with the observations made by Priestley and Irving 

 (1907), ZiRKLE (1926) and Wieler (1936), according to which the 

 colouring matter is accumulated in the cortex and is lacking in the 

 centre. 



As only the grana contain the pigment, they alone show the fluor- 

 escence of chlorophyll (Heitz, 1936b; Metzner, 1937), appearing 

 bright red, whereas the stroma remains dark. In this way the hetero- 

 geneous distribution of the chlorophyll can be proved indubitably, 

 even in what appears to be optically homogeneous chloroplasts. 



MoMMAERTS (1938) is of Opinion that the minute green particles 

 occurring in infusions of ground leaves (Noack, 1927) are isolated 

 grana, which he subjects to chemical analysis. Gjr.anick (1938) and 

 Menke (1938b), however, succeeded in obtaining undamaged chloro- 

 plasts from the leaves for chemical examination. 



Strugger (1950) has discovered that the small amoeboid un- 

 diflerentiated proplastids which exist in dividing meristematic cells 

 already contain a single primary granum. This minute disc multiplies 

 by auto-reproduction. When two grana are formed in this way, the 



