Abstract
The phenomenon of aging of leaves in autumn was studied in three deciduous trees, viz. Acer ginnala Maxim., Populus tremuloides Michx. and Rhus typhina L. The leaves were collected at three stages of development: at maximum summer development, at early senescence, and at a full senescence. Electron microscope studies of fully matured leaves generally showed lens-shaped chloroplasts fairly uniform in shape and size with extensive granalamellae, large starch granules and a few small lipid globules. Green, partly senescent leaves had somewhat smaller chloroplasts with large lipid globules within the reduced stroma-lamellae; virtually no starch granules were present. Fully senescent, brightly coloured, autumn leaves showed a definite decrease in size and number of chloroplasts. These plastids had lost their typical summer structure, were small and cylindrical, and contained densely packed globules that replaced the membrane structures. The presence of the carotenoids in these globules probably contributed to the autumn pigmentation of the leaves. Not all leaf cells followed the above described pattern of senescence at the same rate. The chloroplasts of palisade cells degraded earlier than those in the mesophyll cells.
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