GROWTH. 



185 



short, in diminished hght 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 ("^ 167) 

 is p)Oorly developed. 



263. Light and temperature. — The combined variation 

 of light and temjjerature 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 



i,0 mm 

 1,5 

 1,0 

 0,5 



9 11 1 3 



11 1 



N 



Fig. 186. — Curve show'ing the daily period in the gro\\'th 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 cune is drawn by taking the record from an aux- 

 auometer and laying off on the \-ertical line for each intenal the grovlh shown. The 

 points are then joined. It will be obsen'ed 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 thennometer varied 

 from iS° to 22° C. — .\fter Frank. 



minor fluctuations in temijerature, as well as the generally 

 higher temperature during the day and lower during the night. 



