CHAP. X. 



APOGEOTROPISM. 



501 



Fig. 187. 



upwards, and rose during the first 3 h. 10 m. in a nearly straight 

 line (Fig. 187); but it was not 

 possible to trace the upward move- 

 ment on the vertical glass for the 

 first 1 h. 10 m., so that the nearly 

 straight line in the diagram ought 

 to have been much longer. During 

 the next 11 h. the hypocotyl circum- 

 nutated, describing irregular figures, 

 each of which rose a little above 

 the one previously formed. During 

 the night and following early morn- 

 ing it continued to rise in a zigzag 

 course, so that apogeotropism was 

 still acting. At the close of our ob- 

 servations, after 23 h. (represented 

 by the highest dot in the diagram) 

 the hypocotyl was still 32 from 

 the perpendicular. There can be 

 little doubt that it would ulti- 

 mately have become upright by 

 describing an additional number 

 of irregular ellipses, one above the 

 other. 



Apogeotropism retarded l>y Helio- 

 tropism. When the stem of any 

 plant bends during the day towards 

 a lateral light, the movement is 

 opposed by apogeotropism ; but as 

 the light gradually wanes in the 

 evening the latter power slowly 

 gains the upper hand, and draws Bmssica okracea: apogeotropic 

 the stem back* into a vertical movementof hypocotyl, traced 

 position. Here then we have a 

 good opportunity for observing how 

 apogeotropism acts when very 

 nearly balanced by an opposing 

 force. For instance, the plumule 

 of Tropceolum majus (see former 

 Fig. 175) moved towards the dim 

 evening light in a slightly zigzag 

 line until 6.45 P.M., it then returned on its course until 



on vertical glass, from 9.20 

 A.M. Sept. 12th to 8.30 A.M. 

 13th. The upper part of the 

 figure is more magnified than 

 the lower part. If the whole 

 course had been traced, the 

 straight upright line would 

 have been much longer. Figure 

 here reduced to one-third of 

 the original scale. 



