448 .PHYSIOLOGY OF GROWTH. 



lightly running pulley R, over which passes a thread. At one 

 end of this is suspended the small glass Sch ; to the other end of 

 it is attached a hook. The light pointer Z, which moves over 

 the graduated arc G, is somewhat widened at the base. Here is 

 attached the hook above mentioned, and also another which sup- 

 ports a small weight to balance the glass Sell when it dips into 

 water contained in a suitable vessel placed below it. We now 

 suspend from the hook a further weight, e.g. about 1 gr., if we 

 experiment with seedlings of Yicia Faba. The seedlings are suit- 

 ably attached by means of pins to a piece of cork fixed in the 

 water vessel. The downward growing mainroot, whose tip is 

 introduced into Sch, exerts on this a pressure, which forces it 

 down in opposition to the weight attached to the other end of the 

 thread, and the pointer Z consequently changes its position. 1 



We know and we shall return to this point later that the 

 geotropic curvatures (apart from the geotropic movements of 

 variation) are due to processes of growth. Most plants do not 

 grow when deprived of free Oxygen, and therefore, in a medium 

 free from Oxygen, the geotropic nutations also are arrested. This 

 is easily determined by means of the apparatus illustrated in Fig. 

 147. Into the glass cylinder we bring a thin strip of wood jB, soaked 

 by boiling in water, and on this, by means of pins, we fasten seed- 

 lings of Pisum or Phaseolus or epicotyls of the latter plant, or, e.g., 

 scapes of Taraxacum, St, bearing flower buds. The regions cap- 

 able of growth must be free and horizontal. The cylinder is 

 tightly closed with a cork through which pass two glass tubes 



FIG. 147. Apparatus for proving that plant structures do not undergo geotropic curva- 

 tures in absence of Oxygen. 



bent at right angles. One of these, a, is connected up with a 

 Hydrogen apparatus ; the other, 6, is the exit tube for the gases 

 leaving the cylinder. For an hour we [pass a stream of moist 

 Hydrogen through the apparatus, but no geotropic curvature can 

 be perceived when the cylinder, after standing for a time vertically 

 (for about half an hour after starting the current of Hydrogen), 

 is laid in a horizontal, position. In a similar cylinder, seedlings or 



