1102 BIOLOGICAL EFFECTS OF RADIATION 



for the first six or eight weeks, but after this time the development of 

 chlorophyll is retarded and the plants become yellow and stunted. 

 Chiefly on theoretical grounds McHargue concludes that manganese 

 plays a role in the synthesis of proteins, functions as a catalyst in plant 

 metabolism, and also acts with iron in the synthesis of chlorophyll. 



Kostychev (33) demonstrated that excess of certain minerals in the 

 soil may cause diminution of the chlorophyll content of plants. He 

 found that fruit trees which had become accustomed to soils poor in lime 

 showed chlorosis when grown on soils which had been hea\dly irrigated 

 with lime water. 



It has long been known that plants are unable to build up chlorophyll 

 without the presence of oxygen. Friedel (20), using Lepidium sativum 

 seedlings, found that in one-half atmospheric pressure normal greening 

 took place, but under one-fifth normal pressure there was a greatly 

 diminished formation of chlorophyll. Similar results were obtained with 

 Phaseolus muUiflorus. It should be pointed out that this question is 

 closely related to the one raised by Pfeffer, Liro, and Lubimenko as to 

 the part the living cells play in the formation of chlorophyll. Interesting 

 relations between nitrogen and chlorophyll content of leaves in autumn 

 were recently elucidated by Hilpert and Heidrich (26). 



Palladin (51) pointed out that carbohydrates are essential to the 

 formation of chlorophyll since etiolated leaves which contain appreciable 

 amounts of soluble carbohydrates green rapidly when illuminated, while 

 those which contain little or no soluble carbohydrates remain yellow. 

 If, however, these yellow leaves are floated on a solution of glucose or 

 cane sugar, rapid greening occurs. Senn (66) found that etiolated leaves 

 of corn and beans floated on sugar solutions of different concentrations, 

 under illumination, will not turn green in low light intensities and strong 

 sugar solutions. Strong light, however, counteracts this effect. It has 

 been shown that Chlorella, if glucose is added to the nutrient media, will 

 form chlorophyll in the dark and, if one wished to speculate on this mat- 

 ter, it may be considered possible that any cell which is inherently capable 

 of forming chlorophyll in the light could, if constructed for absorbing 

 soluble carbohydrates, form chlorophyll in absolute darkness. This was 

 apparently not so far from the minds of Monteverde and Lubimenko in 

 their ideas on the relative effects of photochemical and strictly chemical 

 formation of chlorophyll in the living plant. 



YELLOW PIGMENTS ASSOCIATED WITH CHLOROPHYLL 



Yellow pigments are always present in green leaves. In the literature 

 carotene and xanthophyll are usually mentioned. Palmer's monograph 

 (52) gives a detailed account of the carotinoids and related pigments to 

 the year 1922. During the last few years, however, our knowledge of the 

 yellow pigments has increased considerably. Thus, three different forms 



