CHLO'ROPLAST (from Gk. xXwocic, chlOros, greenish-yellow + r Roark, plastos, formed from TrXdaacw, plasscin, to shape). In botany, the protoplasmic body found in green plants and dis tinguished by the green pigment chlorophyll. Chloropla.sts arc developed only in the cells to which a sufficient amount of light penetrates: therefore. in the larger plants they are found only in the cells near the surface. The absorp tion of light by six to ten cell-layers is so com plete that neither chlorophyll nor ehloroplast is developed. Chloroplasts originate from preex istent structures of the same kind. Unspecial ized protoplasmic organs known as plastids (q.v.) multiply by direct division as the cells di vide. Later those plastids in the superficial cells may differentiate into chloroplasts, which also may increase by direct division; those in storage organs may become leucoplasts (q,v.) ; or in appropriate regions they may form ehromoplasts (q.v.) ; or the chloroplasts may later be trans formed into chromoplasts.
The form of chloroplasts is quite various in some of the alga. In Spirogyra there are one to several chloroplasts. Each is ribbon-like, with an irregularly toothed edge, somewhat depressed and thicker along the median line ( thus trough like) and spirally coiled in the protoplasm near to the cell-wall. In Mougeottia the chloroplast is single and plate-like, occupying the axis of the cell. In there are two ebloroplasts, each an irregular mass. with radiating arms reaching to the periphery. All these chloroplasts are peculiar in having one or more pyrenoids (q.v.) imbedded in them. In species of Antho ceros and Selaginella each cell contains a single irregular chloroplast. But in most alga and
mosses, and in all the higher plants the ehloro plasts are several or many in each cell, and have a somewhat flattened globoid or ovoid form.
The structure of the chloroplast, like that of other protoplasmic organs, is not definitely known: but it appears to be vacuolate, i.e. to consist of a firmer colorless portion inclosing many minute spaces filled with chlorophyll: but just how the latter is related to the colorless portion, and whether dissolved or not, is not satisfactorily determined.
The ehloroplasts are capable of moving about the cell. When the protoplasm is rotating, the chloroplasts are swept along with it; but they are capable of independent creeping. When a leaf of sorrel, for example, is exposed to diffuse light, the ehloroplasts arrange themselves on the best illuminated sides (top and bottom) with their broad sides turned to the light. This posi tion is laiown as epistrophe. In direct sunlight they pass to the side walls, standing edgewise to the light, a position known as light apostrophe. A somewhat similar position is assumed in pro longed darkness (dark apostrophe), the chloro accumulating also on the bottom of the cell.
Photosynthesis depends wholly upon the chloroplast and its associated chlorophyll. The deeolorized chloroplast is unable to form carbo hydrates, and chlorophyll separated from the chloroplast is equally functionless. See ETioux; PHOTOSYNTHESIS; MOVEM ENT; PROTOPLASM.