4 THE STRUCTURE OF ECONOMIC PLANTS 



transmitting the heritable characters of the organism; but some 

 instances of cytoplasmic or plastid inheritance have been demon- 

 strated. 



Plastids. — Plastids are the most conspicuous structures in 

 the protoplast and occur in some of the tissues of practically all 

 plants with the exception of certain groups of Thallophytes 

 (Cyanophyceae, and fungi). Like the nucleus, they lie in the 

 cytoplasm and are not known to occupy vacuolar cavities. They 

 are variable in shape, being spherical, ovoid, or discoid; but the 

 size is relatively constant. Mobius Ql^^ found that 75 per cent of 

 the plastids in ii5 species of plants examined had a long axis rang- 

 ing from 4 to 6 ju. Among those which fall within the average 

 range are the plastids of Cucurbita, Gossypium, Lactuca, and 

 Phaseolus; in Beta they are larger, ranging from 7 to 10 /i. 



The manner in which the plastids originate is not entirely clear; 

 but, in most instances, it appears that they arise by the division of 

 preexisting plastids or from proplastids. Randolph (35), working 

 with Zea, was able to observe the behavior and development of 

 proplastids in living cells and noted the formation of mature 

 plastids from minute granular proplastids. Early stages occur 

 in the tips of leaf primordia, transitional ones appear near the 

 apices of successively older seedling leaves not yet exposed to sun- 

 light, and later stages may be found in leaves that are about to 

 emerge from the surrounding sheath. In cells of the mesophyll of 

 well-developed seedling leaves, the chloroplasts have a diameter of 

 7 to 8 /i. 



Whether or not the proplastids are always formed from other 

 cytoplasmic inclusions such as chondriosomes, or arise de novo, is 

 not established. Randolph has concluded that "the question 

 regarding the extent to which the plastids are to be considered 

 permanent cell organs with an unbroken genetic continuity 

 throughout the life cycle must remain an open one"; and Sharp 

 (40) has reasoned that "if plastids represent transformations of the 

 cytoplasm resulting from the localization of certain processes, they 

 may well be expected to differentiate anew as these processes begin, 

 and to preserve varying degrees of permanence depending upon the 

 processes carried on . " On the other hand , Bowen (9), working with 

 the root tissues of several plants including Pisum, Hordeum, Cucur- 

 bita, and Phaseolus, concludes that "it is perfectly clear now that 

 plastids are formed only by the division of preexisting plastids." 



