184 THE PLANT CELL. 



CO2 proceeds most actively under the influence of the rays of the 

 spectrum which are of low refrangibility. 



c and d. The Action of Physical, Tactile, and Chemical Stimuli 

 upon Protoplasm. — The efFect of gravity (geotropism), or a cen- 

 tripetal force, upon protoplasm, is very marked at times. Thus, 

 there arises a directive action which causes root-tips to curve 

 downwards into the soil, even when seedlings are placed horizon- 

 tally ; and stem-structures will, on the contrary, curve upwards. 

 Eoots are thus said to be positively geotropic, whilst stems 

 are negatively geotropic, and the curves shown by these struc- 

 tures under such conditions are known as geotropic curvatures. 

 If seedlings are grown upon the edge of a rotating wheel, 

 the plane of rotation being at right angles to the line of 

 action of gravity, the roots will grow out along the radii 

 of this wheel, or at any rate along the line of action of the 

 resultant of the centripetal force and the force of gravity 

 (Knight's experiment). This shows that a force acting in any 

 direction, which is strong enough to overcome gravity, will 

 determine the directive action of the protoplasm in root-cells. 

 An instrument known as the clinostat is used nowadays for 

 the demonstration of geotropic curvatures ; it consists of a flat 

 wheel which can be kept rotating by means of a clockwork 

 device at any constant speed which is desired.* 



With regard to rhizomes and stolons, the effect of gravity is 

 to produce growth in a horizontal direction. This is known as 

 diageotropism, and the growing apex of a rhizome will, if placed 

 vertically, soon return to the horizontal position. 



The influence of gravity is manifested in all these cases by an 

 increased growth of one side of the organ, and a diminished 

 growth of the opposite side {vide action of light), so that a 

 geotropic curvature results. 



If the cells upon one side of the apex of a quickly-growing 

 stem be more turgid than those on the other, increased growth 

 will take place on that side, and a corresponding curvature of the 

 apex away from that side will be produced ; this curvature is 

 known as a nutation. At times a periodic alteration in the 

 turgidity of the cells all round the apex takes place in succession, 

 and the curve thus produced is a circle, or since growth iu 



* See Detmer and Moore, Practical Plant Physiology. 



