CHLOROPLAST (Gk. xXiJp6c. light green, and /Ozark, formed). The chlorophyll (q.v.) or green coloring-matter of plant-cells is not diffused throughout their mass, but is col lected in certain special bodies known as chloro plasts. These chloroplasts may be of the most varied shapes; they may be granular, as is the case in most flowering plants, or spiral bands, as in Spirogyra, or flat plates, as in Mesocarpus. They contain particles of nutritive material in the form of starch-grains, oil-drops, and highly refractive masses of proteid known as pyrenoids.
Bodies of the same general character as chloroplasts, but differing from them in the ab sence of chlorophyll, are known as leucoplasts or chromoplasts according as they are colorless or pigmented. Chloroplasts, leucoplasts and chromoplasts are all known as chromatophores. In all embryonal cells, all the chromatophores have the appearance of leucoplasts, being small, transparent, highly-refractive bodies of granu lar, spindle-like, or thread-like form. It is because of this common origin that the chro matophorcs, or as they are also called, plastids; are classed together under a single heading.
In those phanerogams that lack a green color, the chloroplasts are replaced by other chromato phores. In the fungi, however, which likewise lack chlorophyll, there are no chromatophores of any sort. The chromatophores of the blue greed, red and brown alga contain chlorophyll, but in addition other pigments, known as phy cocyan in the blue-green alga, phycoerythrin in the red alga, and fucoxanthin in the brown alga.
The chloroplast as the chlorophyll-bearing organ, is undoubtedly that part of the cell which is most directly concerned in the assimilation of carbon dioxide. This process is very imper fectly understood, but is believed to involve the action of an enzyme. (See CHLOROPHYLL).
Ponomarew claims that the chloroplast is liquid, and of a colloidal nature. He explains its relative permanency of shape by supposing that it is in equilibrium with the surrounding cell-contents as to surface-tension. He accounts for the contraction of the chloroplast into a spherical shape when the cell is treated with dilute alcohol as due to a disturbance of this equilibrium.
In most plants the chloroplasts are sensitive to light, and place themselves in the cell in such a manner as to offer the greatest surface possi ble to a slight illumination, but the least surface possible to an intense illumination. Thus the plate-like chloroplast of Mesocarpus presents..its face to a weak light, its edge to a, strong one. The chloroplasts of Lemna teisulca group themselves in planes parallel to the surface of the leaf when moderately illuminated, but perpendicular thereto if the illumination be intense. In all cases the position in the dark is less definitely determined. On account of these light reactions of the chloroplasts, the suggestion has been made that they act in a manner analogous to sense-organs, but this idea of a specific sensi tivity on their part has been opposed by Pfeffer.