GROWTH. 



185 



short, in diminished light may have them well developed, as 

 occurs, for example, in dandelions growing in deep shade. 



In general, light accelerates the growth of leaves in area. 

 Leaves of shoots grown in darkness remain small. 



Light affects not only the external form but the internal 

 structure. In diminished light the cell walls do not thicken 

 normally, and mechanical tissues are weakened. " Laying" 

 of oats and such grasses is mainly due to this cause (fig. 185). 

 In weak illumination the palisade tissue of the leaves (^j 167) 

 is poorly developed. 



263. Light and temperature.- -The combined variation 

 of light and temperature between day and night establishes a 

 daily period in the growth of all plants. The withdrawal of 

 light at night permits an increase in the rate of growth in 

 length, which reaches its maximum in some plants shortly 

 after midnight, in others not until the early morning. During 

 the day its retarding effect diminishes the rate of growth, 

 which reaches a minimum some time in the afternoon. The 



FIG. 1 86. Curve showing the daily period in the growth of a stem of rye. The vertical 

 lines represent 2-hour periods from 5 P.M. of one day to 5 A.M. of the second day, 

 the shaded parts indicating the actual hours of darkness. The horizontal lines repre- 

 sent tenths of a millimeter. The curve is drawn by taking the record from an aux- 

 anometer and laying off on the vertical line for each interval the growth shown. The 

 points are then joined. It will be observed that the maximum rate of growth occurs 

 shortly after the period of darkness (5 A.M.) and the minimum rate after the period of 

 most intense illumination (5 P.M.). During the experiment the thermometer varied 

 from 18 to 22 C. After Frank. 



minor fluctuations in temperature, as well as the generally 

 higher temperature during the day and lower during the night, 



