132 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



Tubular fibrils, about 21 rri/A in diameter and as many as eight 

 or nine in number, run parallel to and beneath the grooves of 

 the pellicle (Fig. 49, PI. XIII). In addition, structures interpreted 

 by Roth as heavy fibers are seen below the pellicular crests. 

 Their diameter fluctuates between 130 and 230 m/x, and alternating 

 regions of varying densities and vesicular enclosures indicate a 

 periodic repeating pattern. They are present under only about 

 half of the grooves seen in any one section, with no evident 

 regularity in distribution. In view of the facts that very few 

 longitudinal sections of these structures have been seen and that 

 these show only short segments, their linear continuity is question- 

 able. Certainly more information is required before any function 

 can be attributed to them. No organelles seen are identified as the 

 mucigenic bodies that occur in rows along the pellicular striae 

 and are demonstrable by appropriate staining techniques in the 

 light microscope. 



To attempt a comparative summary of phytoflagellate ultra- 

 structure based on the very small number of species sampled is 

 obviously a precarious business, but it seems worth trying, if only 

 to tempt future workers to demolish it with more information. 

 Chloroplast structure probably is destined to be understood and 

 categorized as soon as any. The simple, single-disc lamellae of 

 the unicellular red algae and the segregation of discs into dis- 

 continuous piles in the phytomonads would seem to represent its 

 extremes of development among the phytoprotists ; evolution of 

 grana in higher plants from the phytomonad type is probable, but 

 morphological intermediates in this series have not been described. 



It is tempting to ascribe some significance to the general 

 occurrence of cristae in the mitochondria of green algae, as 

 opposed to the microtubules found in all representatives of the 

 brown stock (as well as the great majority of other protozoa). 

 Both the land plants and the metazoa commonly have cristal 

 mitochondria, and for those who believe that metazoan organiza- 

 tion is more likely to have stemmed from green algal colonies 

 than from heterotrophic flagellates or ciliates, this may be a 

 welcome bit of evidence. But some microtubular mitochondria 

 are found both in land plants and in animals (Rouiller, 1960; 

 Novikoff, 1961), and we know little of the physiological meaning 

 of variations in the configuration of mitochondrial membranes. 



