PHYTOFLAGELLATES 117 



flagella of this species and its chlorophyll-bearing relative, 

 Cryptomonas sp., were shown by Brown and Cox (1954) and 

 Pitelka and Schooley (1955) to bear rather long, slender masti- 

 gonemes(Fig. 39, PL XI), pantomenein distribution but apparently 

 less rigid than ochromonad mastigonemes. Hemiselmis virescens 

 (Pitelka and Dougherty, unpublished) likewise has long, slender 

 mastigonemes on both flagella. 



Class Phytomonadea 



The flagellated green algae, called Phytomonadea by zoologists 

 and Volvocales by botanists, are of extraordinary interest to both 

 disciplines because of their approach to multicellular organization. 

 Clearly akin to the higher green algae and presumably to the land 

 plants, they also have figured prominently in speculations on the 

 origin of metazoa. Thus Haeckel's hypothetical blastaea was 

 derivable from a hollow colony of flagellates resembling Volvox 

 (Hyman, 1940), but modern heretics are engaged in vigorous 

 dispute over metazoan origins (see Hanson, 1958; Greenberg, 

 1959). Relationship of the phytomonads to other specific groups 

 among the protists remains obscure. 



Credit for electron-microscope studies of the simplest green 

 flagellates yet investigated is due once more to the energetic 

 British botanists, Manton and Parke. Manton (1959a) made a 

 detailed examination of a minute species until then known as 

 Chromulina pusilla and assigned to a position in the Chrysomonadea, 

 and came to the conclusion, based on pigment analyses as well as 

 morphology, that it belonged more properly in or near the 

 Phytomonadea. Manton and Parke (1960) subsequently redesig- 

 nated the species as Micromonas pusilla and added a new species, 

 M. squamata both to the genus and to their list of electron-micro- 

 scope conquests. An additional small green flagellate, Pedinomonas 

 tuberculata, was also studied. 



Micromonas pusilla is the first phytoflagellate that upon electron- 

 microscope scrutiny has clearly lived up, or down, to its reputation 

 as a uniflagellate species. Except in cells believed to be dividing, 

 sections contain a single basal body only; indeed there is hardly 

 room for a second in the tiny cell, which averages 1 by 1 -5 fi. The 

 posteriorly-directed flagellum arises laterally from the pear-shaped 

 cell body and consists, as noted earlier (p. 42), of a very short 



