The chloroplasts of the higher plants are green, and they vary somewhat in size, but they have a similar structure and form.
Chloroplasts are mostly oval in longitudinal view and rounded in cross-section view. Each chlorophyll grain has an extremely thin outer wall, which encloses the protoplasmic substance, the green granules, a green pigment (chlorophyll), and a yellow pigment (xanthophyll). Frequently the wall includes starch, oil drops, and protein crystals.
Chloroplasts are arranged either in a regular peripheral manner along the walls, or they are diffused throughout the protoplast.
The palisade cells of most leaves are packed with chlorophyll grains. In the mesophyll cells the chlorophyll grains are not so numerous, and they are arranged peripherally around the innermost part of the wall.
Chloroplasts multiply by fission—that is, each chloroplast divides into two equal halves, each of which develops into a normal chloroplast.
Chlorophyll occurs in the palisade, spongy parenchyma, and guard cells of the leaf; in the collenchyma and parenchyma of the cortex of the stems of herbs and of young woody stems, and, under certain conditions, in rhizomes and roots exposed to light. Almost without exception young seeds and fruits have chlorophyll.
In powdered leaves, stems, etc., the chlorophyll grains occur in the cells as greenish, more or less structureless masses. Yet cells with chlorophyll are readily distinguished from cells with other cell contents. In witch-hazel leaf the chlorophyll grains appear brownish in color. Powdered leaves and herbs are readily distinguished from bark, wood, root, and flower powders.
Leaves and the stems of herbs are of a bright-green color. With the exception of the guard cells, the chloroplasts occur one or more layers below the epidermis; but, owing to the translucent nature of the outer walls of these cells, the outer cells of leaves and stems appear green.
Wild cherry, sweet birch, and, in fact, most trees with smooth barks have chloroplasts in several of the outer layers of the cortical parenchyma. When the thin outer bark is removed from these plants, the underlying layers are seen to be of a bright-green color.