426 RADIATION BIOLOGY 



The effect of sugar is dependent on its concentration. Palladin (1918) 

 showed that certain etiolated leaves floated on solutions of low and 

 medium concentrations of sucrose produced chlorophyll when illuminated 

 but failed to do so when the sucrose concentration was 35 per cent. 

 When returned to solutions of 5-10 per cent sucrose concentration, these 

 same leaves greened readily. Trebitz (1905) injected etiolated leaves 

 with sugar solutions of various concentrations and found that with 15 per 

 cent solutions there was no inhibition of greening; with 20 per cent, a 

 visible inhibition occurred; and with 30 per cent (40 per cent for cereals) 

 there was no greening before death. Chlorella luteo-viridis, according to 

 Kufferath (1913), remains completely chlorotic in nutrient media con- 

 taining 7-20 per cent sucrose even though it grows abundantly; with 

 concentrations of 4-6 per cent it produces variegated cultures of yellow 

 and green cells; but with concentrations of 3 per cent or less it forms 

 completely green cultures. 



Chodat (1911, p. 515) considers that the chlorosis induced by an excess 

 of assimilable nutrients may arise from a tendency to saprophytism or 

 obligatory parasitism. 



Although many substances inhibit chlorophyll formation, the result of 

 their action does not come from a single cause. Some act through hin- 

 dering chloroplast development, some by action on specific enzymes, and 

 others througli "osmotic effects" or inducing "saprophytism" — whatever 

 these last terms mean in this connection. 



7. CHLOROPHYLL FORMATION IN THE DARK 



Up to the present we have been discussing almost entirely chlorophyll 

 formation in the angiosperms, which requires light. In other plant 

 groups, chlorophyll can be formed in the dark. This was discovered by 

 Sachs (1859) when he observed that pine seeds, even though germinated 

 in the dark, produced seedlings with intensely green cotyledons. Later 

 many lower plants were found to produce chlorophyll in the dark. 

 Because of this, Schimper (1885, p. 159) asserted that probably in all 

 the lower forms of plants, up to and including the mosses, the ability to 

 form chlorophyll is independent of light. Bittner (1905) questioned this 

 broad generalization because of the known exceptions; however, she 

 recognized that, in the more highly organized plant forms, the ability to 

 form chlorophyll in the dark is often lost. But, as we will see, this ability 

 is often lost also in the lower forms, such as algae. Although the phylo- 

 genetic aspects of this subject present many interesting possibilities for 

 further investigation, they cannot be entered into here. Rather we will 

 discuss some of the physiological aspects that have received attention. 



The question whether algae can grow continuously in the dark for long 

 periods of time and produce chlorophyll has been studied by Artari and 



