CHAPTER VI 

 PLASTIDS AND CHONDRIOSOMES 



PLASTIDS 



Next to the nucleus, the most conspicuous organ held within the 

 cytoplasm of the plant cell is the plastid. Cytologists have long been 

 aware of the important physiological roles played by plastids of various 

 types in the life of the cell, but it is only recently that an added inter- 

 est has been given these organs by the discovery that certain peculiar 

 characters showing definite modes of inheritance are closely bound up 

 with their behavior. Such problems are complicated by the relation 

 apparently borne by plastids to chondriosomes. In the present chap- 

 ter will be set forth some of the more important facts regarding these 

 two classes of cell elements. 



General Nature and Occurrence. Plastids are differentiated portions 

 of the protoplasm, as von Mohl long ago pointed out, ' and represent 

 regions in which certain processes have become localized (Harper 1919). 

 In view of their power of growth and division and their definite relation to 

 certain important physiological functions they are to be regarded as 

 distinct cell organs. 



Although plastids can be found in the cells of both animals and plants 

 they are chiefly characteristic of the latter, where they are present in 

 one form or another in all groups with the possible exception of bacteria, 

 myxomycetes, and certain fungi. They are abundant only in those plant 

 parts which have to do with specialized physiological functions. Within 

 a single cell there may be regularly but one plastid, as in many algse, 

 Anthoceros, and the meristematic cells of Selaginella (Haberlandt 1888, 

 1905) ; or two, as in Zygnema; or a /figher 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 fre- 

 quently related in a definite manner to certain external conditions: in 

 the palisade cells of green leaves, for example, the chloroplasts are found 

 near the upper surface if the incident light is weak, whereas they react to 

 strong illumination by taking up less exposed positions along the lateral 

 walls. 



Plastids may be conveniently classified on the basis of their contained 

 coloring matters. This difference in color, however, is secondary in 

 importance; the fundamental distinction is that based upon the kind of 



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