PHYTOFLAGELLATES 109 



small vesicles referable to the endoplasmic reticulum, and lipid 

 and leucosin droplets. The two elongate lateral chloroplasts 

 contain more or less flexuous lamellae, oriented longitudinally 

 and converging toward the two extremities of the plastid. At the 

 anterior end of one chloroplast is the stigma, a curved plate of 

 ovoid vesicles packed in a single layer and containing a dense 

 homogeneous substance; the walls of the vesicles are continuous 

 with the chloroplast lamellae. Occasional cells show secondary 

 vesicular zones at the posterior end of one or both chloroplasts. 

 No pyrenoids are described, but one of the published micrographs 

 shows a zone in one chloroplast in which the matrix is quite dense 

 and the lamellae regularly spaced, resembling a pyrenoid of the 

 Euglena type (p. 23). 



The nucleus of Chromulina psammobia, located between the 

 chloroplasts in the anterior half of the cell, has a conventional 

 double membrane and central nucleolus. The single, long, motile 

 flagellum inserts at the anterior end of the cell, where the central 

 pair of flagellar fibers originates at a small, dense, flat granule at 

 the level of the cell surface. From one side of the base of the 

 conventional fibrous kinetosome a slender, obliquely striated 

 ribbon extends to the cone-shaped apex of the nucleus (Fig. 36, 

 PL X). No direct contact between this ribbon and the nuclear 

 membrane is evident, rather the ribbon bends and may run a short 

 distance along the nuclear surface before disappearing. The 

 period of the striations is about 42 m/z. Fibers called rhizoplasts, 

 passing from flagellar basal bodies to the nuclear surface, have 

 been described in many flagellates by light microscopists. The 

 ribbon-like filament of C. psammobia is a classical example of a 

 rhizoplast seen at the electron-microscope level; however, it will 

 be seen that very similar fibers in other flagellates may have no 

 detectable association with the nucleus, at least in the non-dividing 

 stages examined. 



On the medial side of the rhizoplast is an elongate, dense mass 

 that accompanies it almost to the nuclear surface. Rouiller and 

 Faure-Fremiet (1958a) suggest that this may be a centrosome, but 

 no observations of mitotic stages were available. A well-defined 

 Golgi zone appears in a constant position between the kinetosome 

 and the nucleus, just lateral to the rhizoplast. 



Close to the kinetosome of the external flagellum, on the side 



