388 PHYSIOLOGY OF GROWTH. 



weather (evening temperature at 11 o'clock still 15 C.), reached 

 a height of 71 cm. 



The energy of growth is a function of the duration of the 

 growth and the rate of growth. The energy of growth of the 

 individual internodes of a stem, and therefore their ultimate size, 

 is not the same. If, for example, pea seedlings are grown for 

 some weeks in darkness, and we determine the length of the 

 different internodes when the stem has quite ceased to grow, it is 

 found that the lower internodes are short, the middle ones long, 

 and the upper ones again shorter.* 



It is further of interest to learn that it is possible to determine 

 the growth which plant structures undergo in a short time, say 

 twenty minutes. Attention has already been drawn, in 153, to 

 the methods to be employed in accurate observations. Here is a 

 demonstration experiment. A thistle tube (T, in Fig. 128) passes 

 through a rubber stopper closing the glass bottle 6r, which con- 

 tains water. In the upper, expanded part of this thistle tube we 

 place a seed of Pisum or Phaseolus, germinated in moist sawdust, 

 and cover it with wet cotton-wool. The mouth of the tube is 

 covered with a small glass plate, Gp. The root of the seedling 

 grows quite well in the moist air surrounding it ; it increases 

 considerably in length. We now place our apparatus on a clino- 

 stat (the apparatus is described and figured below), whose axis is 

 directed vertically, and put the seedling into slow rotation, in 

 order to prevent any heliotropic curvature of its root. Before 

 the clinostat is set in movement, we bring the tip of the root into 

 the field of view of a microscope, directed horizontally by means 

 of a suitable stand. We naturally employ only slight magnifica- 

 tion. Now the clinostat is set rotating, and when, say after 

 twenty minutes, the root again appears in the field of vision of 

 the microscope, it can be demonstrated, especially if the experi- 

 ment is made at a comparatively high temperature (20 to 25) 

 that the root has grown. 



1 See Detmer, Lehrbuch d. Pflanzenphysiologie, 1883, p. 248. 



156. Torsions. 



Torsions are frequently to be observed in internodes, and also 

 in leaves. Beautiful examples of torsion are exhibited by the 

 older internodes of twining stems, to which we shall return else- 

 where, and in plants grown in darkness. Thus, for example, 



