224 PART III. PHYSIOLOGY. [ 53 



the external conditions are favourable to the life of the plant. The 

 following conditions are essential ; a moderate temperature, extremes 

 of heat and cold arrest movement ; a supply of water, all move- 

 ments are arrested by drought ; a supply of free oxygen, in the case 

 of most plants (p. 197) ; 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 photometricutn) has been found to be motile 

 only when exposed to light. Again, various movements of vari- 

 ation, 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. 162), 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 the Sensitive Plant). 



What is exactly the influence of light in promoting movement is 

 not understood, but it is termed the pholotonic influence (p. 162) : it 

 induces, that is, a particular condition, the condition of phototonus, 

 in the protoplasm, without which movement is impossible. 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 : in 

 fact, induced movements are more rapidly arrested by unfavourable 

 conditions than are spontaneous movements. For instance, when a 

 Sensitive Plant is kept in continuous darkness, the leaves first lose 

 their power of responding to stimuli, and later their spontaneous 

 movements cease. 



Irritability may also be abolished by special means. For in- 

 stance, exposure to the vapour of chloroform or ether destroys the 

 irritability of the leaves of the Sensitive Plant. Again, it may be 

 abolished by repeated stimulation, the interval between the stimu- 

 lations being very short. This has been observed in the case of 

 the leaves of the Sensitive Plant and of Dionsea. 



53. Mechanism of the Movements. The ultimate factor 

 in the mechanism of the vital movements of plants, whether spon- 



