CHAPTER IV 



PLASTIDS 



The most conspicuous cytoplasmic differentiations in plants are 

 plastids. They may be looked upon as regions of the protoplast which 

 have become structurally specialized mainly in connection with carbohy- 

 drate synthesis and transformation; hence they serve to call attention to 

 one of the most profound metabolic differences between plants and ani- 

 mals. The total plastid outfit of the protoplast has been called the 

 plastidome by Dangeard."^ 



Occurrence and General Characters. — Plastids are of almost universal 

 occurrence in the tissues of plants, where they are found in one form or 

 another in all groups with the possible exception of bacteria, blue-green 

 algae, myxomycetes, and certain fungi. Within a single cell there may 

 regularly be but one plastid, as in many algae, Aiithoceros, and the meri- 

 stematic cells of Selaginella; or two, as in Zygnema; or a higher number, 

 as in the green tissues of most higher plants. They lie imbedded in the 

 cytoplasm and are often closely associated with the nucleus; they are 

 never found normally in the vacuole. The positions which they assume 

 within the cell are frequently related in a definite manner to certain 

 external conditions, as in palisade and stomatal cells of green leaves. ^ 



Although they may vary considerably in size, Mobius (1920) found 

 that 75 per cent of the plastids in 215 species of plants he examined 

 had a long diameter lying between 4 and 6^. The size may be affected 

 by temperature (0. Hartmann, 1919a). It seems that in some cases there 

 is a relation between the size of the plastids and that of the cells in 

 which they lie (Gates, 1923). Eyster (1929) observed characteristic 

 size differences in certain genetic strains of maize. 



Plastids are usually classified on the basis of the colors of their con- 

 tained pigments. The arbitrary nature of such a classification is 

 apparent when it is realized that two plastids of the same color, or with 

 no color whatever, may be performing very different functions by virtue 

 of unlike substances not visible to the eye; and, further, that the same 

 plastid may be colorless, green, and yellow or red at different stages in its 

 history. A separation on the basis of color is, however, very convenient 

 and usually does correspond to functional diversity. This is shown by 



1 For general accounts of plastids, see A. Meyer (18836), Schimper (1883, 1885), 

 Senn (1908), and Schvirhoff (1924). 



2 Schimper (1885), Senn (1908), Haberlandt (1918), Weber (1925a). 



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