488 FOOD REQUIREMENTS 



bearing facultative photoautotrophs may have proceeded as follows: 

 (1) Certain flagellates lost the ability to use inorganic compounds as 

 the sole source of nitrogen, except in the presence of a suitable organic 

 carbon compound. This type of specialization may or may not have in- 

 volved the loss of chlorophyll in the beginning. (2) The ability to grow 

 in inorganic-nitrogen media was lost completely, so that a single amino 

 acid represented the simplest adequate nitrogen source. (3) The ability 

 to grow in an amino-acid medium was lost, as the degree of specialization 

 approached that of the animal-like flagellates. 



On the other hand, the existence of heteroautotrophic flagellates, 

 which can utilize inorganic nitrogen sources without carrying on photo- 

 synthesis, suggests the possibility that primitive colorless flagellates may 

 have appeared before the origin of chlorophyll. Chlorophyll, with the 

 attendant power of photosynthesis, would thus have been acquired dur- 

 ing the evolution of plant-like flagellates. This hypothesis would gain 

 additional support from the demonstration of chemoautotrophic nutri- 

 tion in flagellates, and the suggestive report of such a phenomenon in 

 Chilomonas Paramecium (Mast and Pace, 1933) is particularly inter- 

 esting. Further evidence may eventually necessitate revision of the cur- 

 rent view that the chlorophyll-bearing flagellates are the most primitive 

 of all the Protozoa. 



Even in adhering to the concept of a primitive chlorophyll-bearing 

 stock, it must be admitted that in the known cases of heteroautotrophic 

 nutrition, the presumed "loss" of chlorophyll has introduced only one 

 new food requirement, a simple organic carbon source (e.g., acetate). 

 A second stage of specialization is represented by such types as the 

 chlorophyll-bearing Euglena deses and the colorless Polytomella caeca, 

 each of which requires a simple organic nitrogen source. The third step 

 in specialization also appears in the Phytomastigophora, and is illus- 

 trated by the chlorophyll-bearing Euglena piscijormis, described by Dusi 

 (1933a) as an obligate photometatroph, and the colorless Hyalogonium 

 klebsii, reported to be an obligate heterometatroph (Pringsheim, 1937a). 

 If the types of nutrition described for these various species are taken 

 for granted, it must be admitted that the presence of chlorophyll is no 

 handicap to progressive physiological specialization. Furthermore, the 

 mere absence of chlorophyll has not necessitated specialization beyond 

 the first degree, although it may be accompanied by the assumption 



