GROWTH. 



l6l 



Fig. 120. — Part of the transverse sections of the stem of rye. A, from a plant grown 

 fully exposed to light: B, from a "laid" plant imperfectly exposed to light, a, 

 epidermis; ^, c, mechanical tissues ; ^if, thin-walled tissues. Highly magnified. —After 

 Koch. 



2,0 mm 



1,5 



1,0 



11 1 3 5 7 9 11 1 

 M N 



Fig. 121. — 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 g 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. 



