32 



FUNDAMENTALS OF CYTOLOGY 



phyll appears to be confined to numerous small platelets, or grana, 

 embedded in the stroma. These grana are sometimes few and large 

 enough to be easily seen, but under some physiological conditions the}" 

 are so small, numerous, and closely packed that the chloroplast appears 

 very finely granular or even homogeneous. Even in these latter cases 

 the stroma, free of grana, may be observed in pseudopodium-like exten- 

 sions or in torn specimens. The fibrous appearance often exhibited by 

 chloroplasts is probably due to a linear arrangement of the grana and to 

 variations in the submicroscopic structure of the stroma itself. 



Fig. 21. — Chloroplasts with pyrenoids. a, in a green alga (Draparnaldia). b, in a 

 liverwort (Anthoceros); pyrenoid consists of numerous small bodies, c, in a green alga 

 (Oedogonium); starch granules are near pyrenoids and elsewhere in the plastid. (c, after 

 F. Schmitz.) 



Chemical tests show that the granum, after the extraction of its 

 chlorophyll, consists mainly of protein and lipide. Attempts have been 

 made to ascertain the exact form of association between these materials 

 and the chlorophyll Avdth the aid of polarized light and fluorescence in 

 ultraviolet light. The results, taken together with observations on the 

 chemical and physical behavior of proteins and lipides, have led to a 

 current hypothesis of chloroplast structure. On this hypothesis the 

 chlorophyll forms a series of monomolecular films on the surfaces of 

 numerous protein layers lying more or less parallel throughout the 

 granum. The chlorophyll molecules have their hydrophile ends asso- 

 ciated with the protein and their lipophile ends with lipide molecules. 



