754 BIOLOGICAL EFFECTS OF RADIATION 



nurseries by Hartley (45), Hartley and Merrill (46), Tourney and Neeth- 

 ling (112), and Li (59). It is possible that such injuries may occur in 

 much larger trees. Sun scald of fruit trees is commonly reported. This 

 is due to excessive heating rather than to too much light. Much so-called 

 winter injury, especially in evergreens, frequently attributed to freezing 

 temperatures, is probably due to the excessive transpiration caused by a 

 warm bright day at a time when the ground is frozen, rather than to low 

 temperature. 



Dexter (27, 28) has shown that light has a profound influence on 

 hardening of plants against cold. Plants deprived of carbon dioxide 

 would not harden under any circumstances. With cold days and warm 

 nights, or continuous cold in darkness, hardening did not occur to any 

 appreciable extent; continuous cold in light, or cold nights and warm 

 days both induced hardening. Any treatment involving low tempera- 

 tures which tended to favor photosynthesis and retard respiration and 

 growth assisted hardening. Tysdal (115) found that alfalfa plants 

 harden better if kept for 16 hr. at 0°C. in the dark, and 8 hr. at 20°C. in 

 the light, than if given a shorter day in light. That shading decreases 

 hardiness has been observed by Auchter and Schrader (5) and several 

 others. Unpublished work of the writer indicates that plants developed 

 in shade are less resistant to drought than those grown in full sunlight. 



MINIMUM LIGHT REQUIREMENTS 



It is beyond the scope of this paper to present a detailed review of 

 ecological studies of the effect of light intensity on plants. A few words 

 may be said, however, about minimum light requirements. 



There is a vast literature dealing with the determination of minimum 

 light requirements of plants as ascertained by ecological methods; i.e., 

 by finding the lowest light intensity under which a given species of plant 

 can exist. Of this work, the investigations of Wiesner (120) are the most 

 extensive. They cover a period of 30 years and include plants of both 

 hemispheres, from tropical to arctic conditions. This work cannot be 

 taken up in detail here, but it is well to point out that the methods used 

 by him and many other investigators are accurate enough to show only 

 general trends. For instance, Wiesner points out that plants in warm 

 regions require less light than the same species growing farther north or at 

 higher altitudes. This, however, is a response to temperature rather 

 than light. Also workers have come to the conclusion that the light 

 requirements of a plant increase with increasing age, and with decreasing 

 soil fertility and soil moisture. Such conclusions must be put to labora- 

 tory test and critical analysis before they can be accepted. Furthermore, 

 the ability of a given plant species to become established in natural shady 

 habitats is often determined by conditions favorable for seed germination 

 and other factors quite independent of light. For further discussion of 



