760 PART IV. — THE PHYSIOLOGY OF PLANTS. 



the directive influence of light and of gravity. The influence of light can be 

 easily climated by performing the experiments in darkness, or by rotating the 

 plant so that each part of the member under observation is exposed for equal 

 lengths of time to rays of light falling upon it in all directions, so that the helio- 

 tropic effect is the same on all sides, and no curvature is produced. The influ- 

 ence of gravity can likewise be eliminated by slowly rotating the plant, so that 

 each side of the organ is placed for equal times successively in all possible 

 positions with respect to the vertical ; the geotropic effect is therefore the 

 same on all sides, and no curvature is produced. The instrument used for this 

 purpose is the clinostat (see p. 752). 



§ 16. Conditions of Movement. Inasmuch as the move- 

 ments under consideration are vital, they can only take place 

 when the external conditions are favourable to the life of the 

 plant. The following conditions are essential ; a moderate tem- 

 perature, extremes of heat and cold arrest movement ; a supply of 

 water, all movements are arrested by drought ; a supply of free 

 oxygen, in the case of aerobiotic plants (p. 722) ; and, in some 

 cases, exposure to light of a certain intensity. 



The importance of exposure to light as a condition of movement 

 requires special consideration. It has been ascertained to be 

 essential to movements of the most different kind. For instance, 

 a Bacterium (^Bacterium photometricum) has been found to be 

 motile only when exposed to light. Again, various movements of 

 variation, such as those of the foliage-leaves of Mimosa, etc., do 

 not take place unless the plant either is, or recently has been, 

 exposed to light. But the most important case is the arrest 

 of growth of dorsiventral members when kept in darkness. For 

 example, if a potato-tuber sprouts in a dark chamber, the produced 

 shoots have excessively elongated internodes (see p. 674), but very 

 small leaves ; the growth of the leaves is arrested in darkness. 

 On the other hand, intense light retards movement (e.g. its action 

 on growth) or altogether arrests it (e.g. arrest of spontaneous 

 movement of the leaves of tlie Sensitive Plant). 



What is exactly the influence of light in promoting movement 

 is not understood, but it is termed the phototonic influence (p. 

 674) ; it induces, that is, a particular condition, the condition of 

 phototonus, in the protoplasm, without which movement is im- 



( possible. [It appears that the rays of low refrangibility (red- 

 yellow) are most favourable for the phototonic condition. 



Irritability also depends upon the above essential conditions : 



I in fact, induced movements are more rapidly arrested by un- 

 favourable conditions than are spontaneous movements. For 



