156 



OUTLINES OF PLANT LIFE. 



the spool-wheel will revolve smoothly but have no unnecessary play. 

 On the. board, with hole for glass tube as a center, mark an arc of go de- 

 grees. The radius of the arc should be a multiple of the radius of wheel. 

 Divide arc into half centimeters. Attach wheat straw to wheel as a 

 pointer. 



To the tip of a growing seedling bean fasten a thread by a slip noose. 

 Pass thread over wheel once and to its free end attach a light weight, 

 just enough to turn wheel and pointer when plant is lifted. Set pointer 

 at o and at intervals read the multiplied growth. By taking observations 

 at regular intervals determine the rate of growth of stem for a week. 

 What regular variation can you discover ? 



216. Grand period of growth. ^The entire duration of 

 growth of an organ is known as its grand period of growth. 

 The growth is not uniform, but is at first very slow, increasing 

 gradually, and then more rapidly, to a maximum, from which 

 it falls rapidly, and then more gradually, until it ceases en- 

 tirely. The accompanying curve (fig. ii8) represents the 



T- 



t 



f'i 



li 



12 16 20 



Fig. ii8.— Curve representing the rate of growth of an internode of .crown imperial for 

 each day during the grand period— in this case 20 days. The height ol each vertical 

 line where it intersects the-curverepresents the total growth for the corresponding 24 

 hours. The numbers indicate days. The maximum growth occurred on the 6th day 

 —After Sachs. 



course of growth in length of a short section of a stem. 

 Growth, however, is not uniform from day to day or from hour 

 to hour. If the line should be drawn so as to show these 

 variations it would be irregularly zig-zag, but would follow 

 the same general course as the smooth curve. (See ^ 222.) 



