258 FINE-STRUCTURE OF PROTOPLASM 11 



errors in interpretation, since a pile of grana consists of only about 

 8 microscopic discs (Strugger, 195 i). 



The submicroscopic lamellae must consist of protein. In the living^ 

 state, the lipids in the grana were probably located between these 

 protein layers. If this picture can be substantiated by further research^ 

 the grana of the chloroplast would represent a layered composite body 

 with alternating protein and hpid lamellae. The chlorophyll is closely 

 associated with the grana lipids, because it emigrates together with 

 them; on the other hand, Menke (1938c, 1943) points out that chloro- 

 phyll migrating with the Hpids imparts conspicuous dichroism to the 

 myelin tubes, lacking in the profile of the chloroplast. Hence a simple 

 combination of hpids and chlorophyll is excluded, which is a further 

 argument in favour of the existence of a chromoprotein. 



From Fig. 13 id it may be concluded that this chromoprotein is 

 arranged in layers. If this conception of the arrangement of the 

 chromoprotein be correct, the principle of laminar surface develop- 

 ment can be consistently pursued from the molecular to the macro- 

 scopical region. The molecular layers compose the discoid, sub- 

 microscopic to microscopic grana (Fig. 130a, p. 255); these, again, 

 lie in layers in discoid or laminar chloroplasts and finally the chloro- 

 phyll grana are exposed to the light, again in foliar laminae. 



Tracing thus a given morphological principle through several 

 orders of magnitude, we are provided with an interesting counterpart 

 to fibre structure, in which linear development plays a similar part. The 

 laminar series: molecular layer/grana/chloroplast/foliar laminae may 

 be compared with the linear series : chain molecule/microfibril/fibre/ 

 fibre bundles of the pericycle. It should be emphasized that in both 

 cases the form birefringence has been the key to the submicro- 

 scopic structure, viz., the discovery of the rodlet birefringence in 

 fibres and of the platelet birefringence in chloroplasts. 



Chloroplastin and the unit of assimilation. The definite estabHshment of 

 the grana as the only loci in the chloroplast containing chlorophyll, 

 calls for a discussion concerning the biochemical concept of chloro- 

 plastin. There is no doubt that the grana represent a high concen- 

 tration of chlorophyll. According to Granick (1949), the chloroplast 

 of spinach contains only 40-60 grana, 0.6 ^ in diameter and 0.08 fjt 

 thick. Since in some instances it has been possible to photograph the 

 grana in profile with the fight microscope (Heitz, 1932), this sub- 



