880 EXTERNAL AND INTERNAL FACTORS CHAP. 26 



species (even when s^'own under apparently identical conditions) may differ 

 in photosynthetic production by a factor of 2 or 3. Effects of age appear 

 not only in the higher plants (see the comparison of young leaves with ma- 

 ture leaves in table 28.V, after Willstatter and Stoll, 1918) but also in cul- 

 tures of unicellular algae (c/. fig. 26.10; van Hille 1937, 1938, Wassink 

 and Katz 1939 and Pratt 1943). We cannot hope to eliminate these 

 internal factors in the study of photosynthesis, but it is possible to keep 

 them a,]iproximately constant, at least for the duration of an experiment. 

 Furthermore, there is not much point in treating these internal factors as 

 mysterious ''plasmatic stimulations" or "inhibitions." It is reasonable to 

 expect that some of them will be traced to accumulations or ehminations of 

 certain chemical compou7ids — catalysts, poisons or metabolites — while 

 others will be found to be connected with changes in the physical structure 

 of the photosynthetic apparatus, for example, with the swelling or shrink- 

 ing of the protoplasm or changes in permeability of cell membranes. In 

 chapter 33 we will discuss the most extensively studied example of the ac- 

 tion of "internal factors"— the induction phenomena, and will attempt to 

 mterpret them as a result of deactivation, in the dark, of certain catalysts 

 required for photosynthesis, combined with the accumulation of metab- 

 olites possessing narcotizing properties. 



6. Aging and Self-Inhibition 



The aging effect, in Chlorella in particular, shown in figure 26.10, has 

 been correlated by Pratt (19431'^) Avith the gradual accumulation of a 

 growth-inhibiting substance of definite chemical and biological properties. 

 Since this is the first case in which an "internal factor" in photosjmthesis 

 was identified as a chemical entity, the observations of Pratt will be 

 described in some detail. 



The production of a growth-inhibiting factor in Chlorella cultures was 

 discovered by Pratt in 1940 (c/. chapter 25, page 833). Later (1942) he 

 observed that this "factor" is a substance that can be extracted from "aged" 

 cells, and whose action can then be demonstrated on young cultures. Its 

 molecules are ~ 15 A in diameter; it is soluble in 95% ethanol, ether, pe- 

 troleum ether and water and is destroyed by heat. It is more effective in 

 neutral than in acid solution, and can more easily be extracted from alka- 

 line than from acid aqueous solutions. It has considerable antibiotic 

 effect on bacteria. 



The water-soluble growth-inhibiting substance (prepared by extracting 

 dried cells of 50-60 day old Chlorella vulgaris cultures with 95% ethanol, 

 evaporatmg to dryness and extracting with water) was tested for its effect 

 on photosynthesis of Chlorella cells from 4 day old cultures (Pratt 1943). 



