ORGANIZED CELL-CONTENTS. 
13 
dissolved and the contents unite to form a shapeless 
mass in which the color changes to a dark green or 
brownish green, owing to the action of the cell-sap 
acid. The plastids may, however, retain much of 
their original color, and even shape, if the plant after 
being gathered is quickly dried, either in a current of 
dry air or between sheets of dry bibulous paper. If 
the temperature of the surrounding media be higher 
than the ordinary, a darkening of the specimen is 
likely to occur. If, on the other hand, the drying is 
performed in direct sunlight, the leaves will become 
light green or yellowish green, due to the transforma- 
tion of the chlorophyll or green coloring substance 
into etiolin. 
There are also seasonal changes in the color of the 
leaves, as in late summer, when the C0 2 assimilation 
process is less active or becomes suspended. The 
leaves change to a yellow, as in Liriodendron, or red, 
as in the maples, sumachs and others. This change 
in the color is due to the fact that the chlorophyl 
pigment is reabsorbed by the plant, and other pig- 
ments which were associated with it thus become con- 
spicuous. The proportion of chlorophyl present in 
the plant in June and in late summer is about twenty- 
five to one. While the yellow color of autumn leaves 
is due to the yellow pigment, xanthopliyll remaining, 
the red color is generally due, apparently, to the action 
of light and air upon the various tannin compounds, 
producing the tannin-reds. 
Chromoplastids. — In many cases, as in roots, like car- 
rot, or flowers and fruits, which are yellowish or 
orange-colored, there is present a corresponding pig- 
ment, which is contained in a narrow elongated 
plastid. Some of these pigments, as the carotin in 
carrot, have been isolated in a crystalline condition. 
