1922] RANDOLPH—MAIZE 351 
Here the proplastids in general are larger and more numerous, and 
often faintly yellowish green. This appears to indicate that the 
first steps of chlorophyll elaboration are initiated in these cells. 
Although not any of the plastids are normally green in most of 
the tissue of the leaf blade, there has been found at the extreme 
tip of the albino seedling leaf a limited region in which the cells 
contain green chloroplasts (figs. 17-19). In such a region, even 
within a single cell, are found transitional stages between colorless 
partially mature plastids and those which are fully green (fig. 18). 
Chlorophyll, therefore, is not entirely absent from the albino 
seedlings of the ‘‘Mendelian white” strain. In these albino 
seedlings, however, there is never developed a sufficient amount 
of chlorophyll to enable the plant to live beyond the seedling stage. 
Microscopic examination shows clearly that the initial structural 
basis for plastid development is present, chloroplasts which are 
normal in appearance being actually found in some cases. This 
latter condition never becomes general throughout the plant. 
It therefore seems clear that in this case failure of the plant to 
become green is not to be explained as the result of an absence of 
plastids or plastid primordia. 
MENDELIAN VIRESCENT 
A third category is ‘Mendelian virescent.” In this strain 
the affected plants in the young seedling stage resemble those of 
the ‘‘ Mendelian white” strain. The seedling leaf is at first white, 
but later nearly the entire leaf becomes green, the color deepening 
rapidly. Later formed seedling leaves are somewhat greener in 
the early stages, while leaves formed toward maturity are entirely 
green from the start. 
In the virescent plants the early stages in the development of 
the plastids are similar in all respects to the corresponding stages 
in a normal green plant. So far as it is possible to tell, the number, 
size, and development of the proplastids is identical in the two cases 
(figs. 20-23). In the tips of the young leaves, as they increase in 
size and become exposed to sunlight, the growth of the proplastids 
and the development of chlorophyll proceed as in green plants 
(figs. 24-26, 32). In the main portion of the leaf blade, on the 
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