Abstract
1. When leaf and bark tissues ofPinus strobus were fixed in osmium, embedded in methacrylate, and sectioned, winter-type chloroplasts appeared as distinct, intact entities under the electron microscope, although often closely appressed to one another in the folds or sides of the cells. 2. Grana and lamellae were clearly visible in winter as well as in summer; the principal change from summer to winter appeared to be the loss of starch and perhaps also “fat droplets”. 3. The larger size of the summer chloroplasts is apparently the result of starch grain growth, but may also be caused by partial vacuolization of chloroplasts in summer. Permanganate-fixed bark cells contained unvacuolated chloroplasts in summer. 4. In winter, a network or reticulmn occurred in all living cells, apparently emeshing mitochondria as well as chloroplasts, and in sieve cells extending through the plates. In summer, this reticulum appeared to be less extensive than in winter.
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