EFFECTS OF LIGHT AND TEMPERATURE ON PLANT GROWTH 559 



generally tied up with synthesis. In all cases polarity is tied up with 

 structure. 



This tie-in of structure with growth makes it extremely important 

 to analyze the growth process with agents that will not destroy the 

 structure. An effective analysis of growth can only be carried out if we 

 divide the over-all process into individual processes. In much hormonal 

 research we apply a disruption of the different parts of an organism on 

 a minor scale. In a biochemical approach, we usually are dealing with 

 almost total destruction. Therefore, often in hormonal research, and 

 usually in biochemical research, we have lost the directional or struc- 

 tural aspects of growth. 



With light we can probe into the cell with no destruction and at 

 the same time can be highly specific in what we accomplish. This is 

 because a pigment absorbs only certain portions ( wave lengths ) of the 

 light. Thus the energy absorbed by a given pigment can be transferred 

 to specific processes inside the cell, and it is interesting to note the 

 number of cases in which the absorbed energy is tied up with polarity 

 —e.g., in phototropism. A short review of the different light-absorbing 

 systems in plants shows that: 



1. Chlorophyll absorbs both in the red and the blue regions of the 

 spectrum, and the absorbed light can be utilized for synthetic processes 

 in which there is no distinction between the red and the blue rays. 

 That is, the energized chlorophyll molecule arrives at the same condi- 

 tion whether it has absorbed red or blue light. 



2. In phototropism the absorbing pigment has absorption charac- 

 teristics very closely similar to carotene. The phototropic action curve 

 shows in the visible spectrum exactly the same action maxima as the 

 absorption spectrum of carotene. There is a possibility that a pigment 

 with absorption characteristics similar to riboflavin is involved in spe- 

 cial cases of phototropism, such as the base response of Avena cole- 

 optiles. The light absorbed by the carotene sets up a polarity in the 

 phototropically-sensitive organ, which then is able to transport auxin 

 laterally, and this polarity persists for several hours after the illumina- 

 tion. 



3. The photoperiodic pigment, phytochrome, is involved in a 

 remarkably large number of different processes, enumerated by Hen- 

 dricks. In all cases a pigment with a main absorption peak in the red 

 and only very minor absorption in the blue is involved. In every case 

 studied, the pigment is effective in very low concentrations, so that very 

 low light intensities will saturate this pigment. In this respect it agrees 

 with the phototropic pigment. The photoperiodic pigment is tied in 

 more or less directly with the growth process, whereas the phototropic 

 pigment is tied in with polarity, causing a redistribution of auxins. This 

 makes it possible to understand the discrepancy that red light specifi- 



