ELECTRON" MKUOSCOPY 1(33 



greater tendency for swelling than the grana; round protrusions on the 

 chloroplast surface can be noticed even in 0.5 il/ sucrose. This is not due 

 to myelin formation, since the latter recjuires a separation of lipides from the 

 proteins, and does not occur in neutral sugar solutions; rather, the lipo- 

 proteids swell as a whole. This swelling pi-oduces l)ubbles of widelj^ vary- 

 ing size; after their surface had solidified, the bubbles collapse, and ap- 

 pear as folded membranes. When such a large collapsed bubble covers the 

 whole chloroplast, it appears as a chlorc^plast membrane. 



Unlike the collapsed bubbles, the myelin figures [cf. fig. 32A.o) do 

 not fold on the object holder. 



As the swelling proceeds, the grana join in it, and after some time may 

 cease to be visible as such, their proteins having been incorporated in tlie 

 bubbles and the collapsed precipitation membranes. 



(b) Shape, Dimensions and Composition 



Several estimates have been made of the total volume occupied by the 

 grana and the stroma, and the absolute amounts of proteinaeeous and 

 lipide material in these two parts of the chloroplast. The conclusions varied 

 rather widely, an imDortant source of uncertainty being the fact that size 

 estimates made by measuring out electron micrographs apply to dry ma- 

 terial, in which the different parts of the chloroplast had shrunk by un- 

 known (and undoubtedly different) factors. 



In making these estimates, one also has to keep in mind the above- 

 described variations in the size of grana from species to species, and their 

 change with the physiological state of the cell. The following calculations 

 (Rabinowitch 1952) apply primarily to chloroplasts of the higher plants. 



Typical mature chloroplasts of higher plants contain from 20 to more 

 than 200 cylindrical grana, 0.5-1 /x in diameter and 0.1-0.2 ^ in height 

 (measured in the dry state!). The average volume of such a dry granum is 

 about 0.05 u'; that of one hundred of them 5 jul This is about 15% of the 

 volume of a fresh chloroplast (about 30 ju^)- However, the grana dimen- 

 sions — the height, in particular — undoubtedly are larger in "live" grana, be- 

 cause of the loss of water (and probably also of fluid lipides) during the dry- 

 ing in vacuum. Therefore, the part of the chloroplast volume occupied by 

 the grana in the living cell undoubtedly is larger than 15% — but how much 

 larger we do not know. Frey-Wyssling and Miihlethaler (1949) used a 

 figure of 50% by weight — which means <50% by volume, because of the 

 higher density of the grana compared to the stroma. In a later paper, Frey- 

 Wyssling (1949) used a much lower estimate (only 6% of the total chloro- 

 plast volume occupied by the grana) ; but this figure seems to be incom- 

 patible with the large amount of chlorophyll that has to find place in the 

 grana {cf. below). 



