CRYSTALLIZATION AND STABILITY 



1783 



terial forms sheets only a few molecules thick, which have the tendency of 

 rolling into cylinders upon drying. The extreme thinness of the crystals 

 explains why their diffraction patterns (Fig. 37B.7) are less sharp than those 

 of crystalline ethyl chlorophyllide, in which the crystals (although, they, 

 too, have the tendency to grow mainly in two dimensions) reach much 



A IJUL 



w^sstiiaa 



C IJU 



B IJLX 



D IjJL 



Fig. 37B.6. Electron micrograph of crystalline chlorophyll a (after Jacobs, V'atter 



and Holt 1954). 



greater thickness. Fig. 37B.8 shows microcrystals of ethyl chlorophyllide 

 a of different size. Fig. 37B.9 is a model of their crystal structure, as sug- 

 gested by Hanson (cf. Chapter 16, p. 448). 



The difference in the x-ray diffraction patterns indicates that Hanson's 

 chlorophyllide model does not apply to chlorophyll, but no definite struc- 

 ture can yet be proposed for the latter. The diffraction pattern of methyl 



