The development and structure of pyrenoids in Bulbochaete hiloensis.

Chloroplasts in zoospore mother cells, zoospores, and sporelings of Bulbochaete are shown to possess spherical granular areas which are interpreted as incipient pyrenoids, which arise de novo within the stroma. Very young pyrenoids are often seen adjacent to pale, fibril-containing areas of the chloroplast. Later, the incipient pyrenoids are often associated with a single thylakoid and become invested with starch grains and cytoplasmic channels as they develop into forms morphologically indistinguishable from the mature pyrenoids of the vegetative cell. In young oogonia, degenerating pyrenoids are seen to possess much starch. Infrequently, the matrix of a degenerating pyrenoid is invaded by the stroma, its dimensions decreasing to those of an incipient pyrenoid. The appearance of young pyrenoid-like structures within lateral outgrowths of the chloroplasts of young oogonia and sporelings may represent a further method of pyrenoid formation.