116 LIGHT, VEGETATION AND CHLOROPHYLL 



increased, the rate of photosynthesis increases up to a certain 

 point, then remains stationary or even diminishes. The law 

 of the minimum explains this by the deficiency of a factor 

 other than the illumination. We have seen, for instance, that 

 in an atmosphere enriched in CO 2, the illumination can with 

 advantage be raised to a higher intensity than it can be in a 

 normal atmosphere. 



In intermittent Hght, on the other hand, the dark periods 

 can be adjusted to allow time for the various factors to come 

 into operation. For example, the poverty of the air in carbon 

 dioxide is compensated by a longer period of contact between 

 the activated elements and the atmosphere, and the slowness 

 of the reactions in a moderate temperature is of little 

 importance if they are given sufficient time to be accomphshed. 



If, therefore, the dark intervals are always long enough, 

 we ought to be able to increase at will the rate of photo- 

 synthesis by more and more powerful flashes, each bringing a 

 greater luminous energy. This increase is certainly achieved, 

 but only up to a certain limit at which the plant tissue is, as 

 it were, saturated with light. Emerson, assuming as before 

 that the chlorophyll cells are limited in number, affirms that 

 at this moment they have all received sufficient luminous 

 energy for each to combine a molecule of CO 2 and a 

 molecule of HgO and give off" a part of the oxygen. 



These chlorophyll cells, which are assumed to contain 

 active chlorophyll and to be capable, at each flash when the 

 tissue is saturated with light, of attacking a molecule of CO 2, 

 by a chemical process provoked by Hght and lasting about 

 A second, are called by Emerson "functional units". 



The number of these functional units is easily determined, 

 since, at saturation, each of them will reduce a molecule of 

 CO 2- Thus a measurement of the number of molecules 

 absorbed per flash will give the number of functional units. 

 For the alga, Chlorella, for example, there is one unit for 

 about 2,500 molecules of chlorophyll. 



Under the microscope, the chlorophyll in each chloroplast 

 is seen to be arranged in minute globules called "grana". 



