THE BIOLOGY OF PLANT GROWTH 441 



by the considerations above and by the elegant measurements of 

 Gaastra ( 1958 ) . 



The efficiency discussed above is of course that to be expected for 

 any area covered with a single layer of leaves. A real plant growing in 

 the field produces several layers of leaves, the lower utilizing the light 

 transmitted by the upper. Since the lower leaves have a smaller in- 

 tensity of light incident upon them, they are correspondingly more 

 efficient energywise. The fact that average light-intensities are less 

 than that of full sunlight, due to clouds and to the rising and setting 

 of the sun, also acts in the direction of increasing somewhat the effi- 

 ciency ( although not the yield of dry matter) of our plant stands. 



Let us, however, pass o\'er these matters and consider instead the 

 central aspect of the problem. It is quite clear that the low efficiency 

 of plants as converters of solar energy is due in the first instance to the 

 fact that the photosynthetic rate becames light-saturated at intensities 

 lower than that of full sunlight. \\'hy is this? Might we hope to create 

 plants which are more effecti\'e at high light-intensities? My own first- 

 approximation anal\'sis of this problem is that plants are inefficient at 

 high light-intensities because they are, so arranged as to be efficient at 

 low intensities. 



Photosynthesis is a multi-quantum process. The energy of several 

 quanta must be absorbed, transmitted to, and concentrated in a central 

 spot to bring about the reduction of a single CO:> molecule. Suppose 

 that we have several chlorophyll molecules wired to this central spot. 

 When each and every one absorbs a quantum within a specified short 

 period, chemistry can be conducted and CO2 can be reduced. But 

 light is very dilute stufi^. Even at an intensity one-tenth that of full 

 sunlight, each individual chlorophyll molecule of the leaf absorbs a 

 quantum, on the average, only about once per second. The time that an 

 enzyme involved in pliotosNuthesis takes to do its work is, however, 

 very much less than this. A chloroplast in which only ten chlorophyll 

 molecules are so arranged as to be able to transfer excitation energy to 

 the enzymatic center will be highly inefficient at low intensities, since 

 only rarely will all ten chlorophyll molecules absorb their needed 

 quanta during the requisite short period of time. The response of such 

 a photosvnthctic system to increasing light-intensity will be of higher 

 than first order. The chloroplast, howexer, is organized in a different 

 manner. As we know from the work of Emerson and Arnold ( 1932 ) , 

 the chloroplast is arranged so that it contains approximately 2,000 times 

 as many chlorophyll molecules as it does reducing centers in which the 

 chemistry of CO2 reduction is conducted. The chloroplast contains 

 what are essentially light-collecting panels, in which the energy of 

 photons absorbed by an\ pigment molecule can be transferred to the 



