736 BIOLOGICAL EFFECTS OF RADIATION 



follows: (a) 1 hr. of light each day, (&) 10 min. of light each day, (c) 2 min. 

 of light each day, and {d) continuous darkness. 



An illumination period of even 2 min. daily was sufficient to cause a 

 profound change in the plants. Vicia Faha and Pisum sativum lost their 

 typical plumular hook, and the lateral leaves began to unfold. With 

 10 min. exposure the leaves had attained a size almost as large as those 

 exposed to 1 hr. of light. Those receiving 1 hr. of light daily developed 

 chlorophyll. Plants grown in the light for a while and then placed in 

 darkness soon developed the typical plumular hook and other features 

 of etiolated plants. The light used by Priestley was much lower in 

 intensity than that used by Trumpf. Both of these experiments clearly 

 demonstrate the need for extreme care in excluding all light from etiolated 

 plants if true etiolation effects are to be obtained. 



MISCELLANEOUS EFFECTS OF DARKNESS 



Light not only has an influence upon the plant organs subjected to 

 its action but also an indirect action on the underground portions of 

 plants. Observing plants grown in nutrient solutions in which both 

 roots and shoots were subjected to illumination or kept in darkness, 

 Probst (79) found that during the first few days shoot growth was 

 greater in cultures in which both root and shoot were in darkness. In 

 every case the shoot or root w^as longer in darkness. Illuminating the 

 shoot caused an increase in root growth, but illuminating the root caused 

 only a slight change in shoot growth. Increasing the humidity stimu- 

 lated both the root and shoot growth. Experiments with oats showed 

 that continuous illumination of the root caused an initial stimulus of 

 shoot growth, followed later by decrease in growth. Exposure to dark- 

 ness after the roots had been in continuous illumination caused a certain 

 stimulation in shoot growth; however, such plants never grew so rapidly 

 as those in which the root had not been illuminated. Long periods of 

 illumination, or even continuous illumination of Avena coleoptiles and of 

 Lepidium shoots produced no visible influence on the growth rate of the 

 root, but with Sinapis, darkening the shoot after a period of continuous 

 illumination caused a decided acceleration in root growth. His final 

 conclusions were that illumhiating the shoots inhibited shoot growth but 

 increased root growth, while illuminating the root inhibited root growth 

 and, in most cases, shoot growth also. 



Some very interesting experiments on the effects of light on the 

 nutrition economy of seedlings and cuttings have been conducted by 

 Reid (80, 81, 82). By choosing seeds and cuttings having a wide varia- 

 tion in the composition of their organic reserves and by cultivating these 

 in nutrient media with and without nitrates, she was able to demonstrate 

 that some of the differences between etiolated and normal plants attrib- 

 uted to the action of light are, in fact, the result of differences in the 



