PROTOZOA AS CELLS 23 



of this material contained loose, twisted, membranous bodies 

 rather resembling mitochondria, which the author believed to be 

 colorless plastid remnants permanently deprived of photosynthetic 

 capacity. Similarly, Sager and Palade (1954) reported that a dark- 

 grown yellow mutant of Chlamydomonas, which, unlike the usual 

 form, cannot synthesize chlorophyll in the absence of light, 

 contained reduced plastids with loose stigmas and no organized 

 lamellae. 



Thus, the integrity of lamellar structure appears to depend on 

 chlorophyll synthesis. In higher plants, formation of chloroplasts 

 from undifferentiated precursors occurs (von Wettstein, 1957), 

 but algae appear to lack this capacity. Analysis of structure and 

 pigment content by a variety of means has permitted Wolken 

 (1956, 1959, 1960) to propose a molecular model for the Euglena 

 chloroplast, with chlorophyll and carotenoid arranged in 

 monolayers on the asymmetric lamellar membrane. 



Gibbs's (1960) analysis of Euglena clarifies the structure of the 

 pyrenoid, a specialized region of the chloroplast in many algae 

 that is believed to function in the elaboration or concentration 

 of reserve products of photosynthesis. Such products usually are 

 found as grains or shells around the pyrenoid. The pyrenoid area 

 of the Euglena plastid has a denser matrix than the rest of the 

 chloroplast although continuous with the latter (Fig. 6, PL II). It 

 evidently is traversed by all adjacent lamellar bands, but each 

 band is reduced to two (rarely one or three) discs as it enters the 

 pyrenoid. Within the pyrenoid, the spacing between the narrow 

 bands is more constant than elsewhere. Pyrenoids in the chryso- 

 monads and phytomonads (Fig. 7, PI. II) similarly consist of 

 discrete regions of denser matrix typically traversed by only a 

 few, rather widely separated, bands that may consist of only a 

 single disc, or by more or less contorted tubules which, however, 

 are continuous with adjacent lamellae. 



An eyespot or stigma occurs in many pigmented phytoflagellates 

 as a small reddish body functioning in photoreception. The 

 pigment is a carotenoid identified in some species as astaxanthin, 

 a /3-carotene derivative. In Chrormdina psammobia (Rouiller and 

 Faure-Fremiet, 1958a) and Chlamydomonas reinhardi (Sager and 

 Palade, 1957) it occupies a differentiated region of the chloroplast, 



