ECOLOGY AND THE FLOWERING PROCESS 27 



to be of minor importance. The amount of moisture or minute 

 particles in the air such as smoke or smog may have an effect on light 

 quality. Ultra-violet light is absorbed by the atmosphere (mostly the 

 ionosphere) and consequently as one goes higher in elevation above 

 sea level, a slightly higher proportion of ultra-violet light is 

 encountered. In general, red wavelengths penetrate these atmos- 

 pheric constituents more readily than shorter wavelengths (for 

 example, blue). Thus when the sun is very low on the horizon in the 

 morning or the evening, and its rays must penetrate a much thicker 

 layer of atmosphere, those reaching the earth may be predominantly 

 red (curve 3, Fig. 3-1). At temperate latitudes, quality also changes 

 slightly throughout the year, since the rays must pass through more 

 air in winter when the sun appears lower in the sky. In the oceans or 

 other bodies of water, light quality changes with depth below the 

 surface. As a rule, the natural changes in light quality are not thought 

 to be very important to the response of living organisms, but interest- 

 ing examples are being discovered, and detailed study may reveal 

 unexpected phenomena. For example, we now know that the 

 response to red or far-red light (see Chapter 7) might explain the 

 growth habits of plants on the forest floor ; thus filtering by chlorophyll 

 of the tree leaves may strongly increase the proportion of far-red 

 light reaching the plants below. Leaf size, stem length, etc., of these 

 plants may be profoundly influenced. 



3. Natural Light Intensities and Flowering 



The response of living organisms to diff'erent light intensities is a 

 fascinating subject for study. Figure 3-2 summarizes a number of 

 these responses. Note that essentially no response is capable of 

 utilizing the maximum intensity of full sunlight. Photosynthesis 

 responds to high intensities, but individual leaves are often photo- 

 synthesizing at their maximum rate when sunlight is only about 

 one-fifth of maximum intensity, although a complete plant conamu- 

 nity may respond photosynthetically to higher intensities. On the 

 other end of the scale, it is most interesting to see that some plants 

 are sensitive to intensities much lower than those which can be 

 detected by the human eye. The situation becomes involved, however, 

 since the human eye must respond within a fraction of a second, 

 while plant responses are measured after exposure for a long period 

 of time. Nevertheless, the plant must be "seeing" these extremely low 



