1036 



BIOLOGICAL EFFECTS OF RADIATION 



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proportionality is not found. Emerson also found (23) "that at low light 

 intensities the same amount of photosynthesis may be carried on by much 

 or little chlorophyll. The chlorophyll is more efficient when dilute." 

 Light saturation, however, is apparent at lower intensities for cells 



deficient in chlorophyll than for those rich in the 

 pigment. 



Light Intensity and Morphological Factors. — 

 It must be realized that photosynthesis is 

 affected by morphological changes, permeability 

 of the protoplast, tropic orientations of the 

 chloroplasts, and by changes in stomatal aper- 

 ture. All of these are influenced by light. This 

 means that the experimental approach to the 

 problem of photosynthesis, especially in land 

 plants, is faced with many complexities, a full 

 discussion of which is beyond the scope of this 

 review (74, 131). 



Time Effect in Photosynthesis. — Harder (38) 

 has shown that when Fontinalis antipyretica and 

 Cladophora are brought into the light, the 

 photosynthetic rate gradually increases for 

 several hours, "induction period." When the 

 rate has reached its peak it then decreases, 

 "fatigue." In contrast to the induction period 

 Fig. 8.— Schematic rep- ^f ggveral hours found by Harder, Emerson (23) 



resentation of possible ^ > ^ 



curves of photosynthetic reports that 15 mm. IS sufficient to attam 

 rates under constant exter- niaximum photosynthesis in Chlorella. The 



nal conditions, showing the i i • 



effect of the light intensity results of van der Paauw mdicate that m 

 under which plants were Hormidium an induction period of from ^ to 



cultivated. The shape of '■ . 



the curve is dependent little more than 1 min. IS necessary, the time 

 upon the ratio of the light depending on the temperature. 



iiiteiisity used in the cultiva- 



tion of the plants to that Later work by Arnold (1) showed that 

 in which the photosynthetic ^^g bchavior of the plant was largely dependent 



rates are determined. With i i- i • • i tt i 



low intensity of cultivation on the light mt(Misity employed. He observed 

 and high intensity of experi- ^hat at 90,000 lux (daylight) the photosyn- 



ment, low ratio, curve 1, ct»7t -i 



is obtained. With high thetic rate of Elodea canadensis decreased 

 y^''^ oo^''® ^^'■^^"'*^- rapidly; at 18,000 lux a gradual decrease; 



(Harder, 38a.) , ' • , 



at 6000 lux an initial increase then a decrease; 

 at 4000 lux an initial increase followed by gradual decrease; and at 2340 

 lux the photosynthesis proceeded at a fairly uniform rate. 



Following his earlier work. Harder (38a) found contradictory results 

 with Fontinalis sp. Instead of the initial increase and subsequent 

 decrease, he observed an initial decrease and subsequent increase in the 

 rate of photosynthesis. An extensive investigation of this discrepancy 



