348 BOTANICAL GAZETTE [MAY 
mesophyll tissue of well developed seedling leaves which are exposed 
to sunlight the enlarging plastids have attained a diameter of 
7-8. In such cells the plastids are closely packed in the parietal 
layer of cytoplasm, and are intensely green (fig. 9). In these cells, 
in addition to the larger plastids there are present a certain number 
of smaller bodies of varying size which cannot be distinguished 
from the proplastids found in the earlier stages. This fact seems 
to indicate that such structures represent partially developed 
plastids and not bodies of a separate category, as some observers 
have believed. Thus not only meristematic cells, but cells of 
mature mesophyll tissue as well, contain proplastids in various 
stages of enlargement, as well as plastids which have attained the 
maximum size and color. 
The condition found in the mesophyll cells of the leaf tissue of 
a mature plant is very similar to that just described for fully green 
seedling leaves. In the former the plastids are slightly larger in the 
exposed portion of the leaf blade, and the number of proplastids is 
somewhat smaller than in the latter. In portions of mature leaves, 
however, which are protected from sunlight by other enveloping 
leaves or leaf sheaths, the development of the proplastids has not 
progressed as far as in the exposed portion of the leaf, regardless 
of the fact that the tissue in question is structurally mature. 
Chlorophyll is almost or entirely absent in such cells. In passing 
outward toward the green portion of the leaf, the proplastids 
become progressively larger, and there is a relatively smaller 
number of more minute primordia. Chlorophyll makes its appear- 
ance with the development of the plastids. Figs. 42-46 illustrate 
the transitional stages found in such a leaf, and the amount of 
shading used in representing the proplastids indicates the relative 
intensity of green color to be found in these stages. 
There appears to be a close correlation between the development 
of plastids and the appearance of chlorophyll in the leaf tissue of 
normal green plants, whether in seedling stages or in the mature 
plant tissue. Maturing plastids 4~5 u in diameter are very faintly 
green, and as they increase in size the intensity of the green color 
also increases, until the plastids have attained their maximum 
dimensions, when they are bright green. 
