EFFECTS OF LIGHT INTENSITY 743 



exposed to 2000 meter-candles for 2 hr. Similar negative results were 

 shown by roots of Avena saliva and Lepidium sativum. 



Jeffs (51), studying the influence of light on the rate of elongation of 

 root hairs of Raphanus sativus and Sinapis alba, found no indication of a 

 light-growth reaction for these cells. The light values used varied up 

 to 3,153,600 meter-candle-seconds. He concluded that such a reaction 

 probably depends upon changes in cell division rather than on changes 

 in cell elongation. 



The true nature of the light-growth reaction has not yet been revealed 

 and probably awaits a more refined experimental technique in order to 

 determine just what happens in the cell during the first few minutes as 

 well as to determine the outward manifestation of this reaction. Appar- 

 ently a change in permeability and also a change in turgor are involved. 

 However, it is safe to conclude that exposures to light, even if of very 

 brief duration, do set up rhythmical changes in growth rate. The type 

 and magnitude of the light-growth reaction are dependent upon the 

 intensity and duration of the light exposure. This reaction is intimately 

 connected with phototropic movements. When acting continuously, 

 light tends to hasten the maturation of plant organs and to produce a 

 concomitant decrease in size. The magnitude of these effects increases 

 with increasing intensity of light. Most roots are apparently insensitive 

 to the light-growth reaction, as are also root hairs. 



THE EFFECT OF LIGHT INTENSITY UPON THE GROWTH AND 

 DEVELOPMENT OF PLANTS 



General Statement. — The gross effects of variations in light intensity 

 upon the growth of plants were well known to early botanists and have 

 been described in the textbooks on general botany, plant physiology, and 

 ecology. Attention in this section will be confined to the deliberate 

 attempts to study experimentally the influence of light intensity on 

 plants. 



Plants grown in very low intensities of light, but of normal daily 

 duration, resemble very closely etiolated plants given short daily expo- 

 sures to Ught, with the important exception that they develop normal 

 or even above normal chlorophyll concentration. However, the stems 

 and roots resemble those of etiolated plants; in fact, when grow^n under 

 extremely low light intensities, the etiolated effects may even be accentu- 

 ated. The stem elongates very rapidly and, as a consequence, is high 

 in moisture content and develops very little mechanical tissue. Root 

 development is at a minimum. As the intensity of the light is increased 

 by slow gradations, leaf development is greatly enhanced, the stems 

 continue to elongate, and root development is kept at a minimum, but 

 the plants begin to take on a healthy normal appearance. 



The internal anatomy of the completely etiolated plant shows only 

 slight differentiation of tissue and poorly developed organs. When the 



