372 VEGETABLE PHYSIOLOGY 



respects, so that no two plants are situated exactly simi- 

 larly though they may be growing side by side. 



The surroundings of an aquatic plant, though in some 

 respects very different from those of a terrestrial one, 

 exhibit the same general features and are subject to almost 

 as frequent disturbances, though a watery environment is 

 more uniform than a sub -aerial one. 



We have considered already the effects which are pro- 

 duced by extremes . of light and darkness upon the be- 

 haviour and the structure of plants. We have however 

 still to examine the rhythmic excitations to which plants 

 are subjected' by the variations of illumination which 

 accompany the alternation of day and night. These are not 

 accompanied in every case by conspicuous responses which 

 can be easily observed, but certain plants exhibit a some- 

 what curious behaviour under these conditions. This is 

 especially connected with the positions of their leaves, which 

 assume different positions during the day and the night. 

 This sensitiveness to the alternation of light and darkness 

 is not however confined to ordinary foliage leaves, but is 

 in many cases shared by cotyledons also. The degree of 

 sensitiveness varies greatly in different plants. 



This form of irritability is manifested in a very marked 

 degree by many plants of the Leguminosce, the Oxalidacece, 

 and a few other Natural Orders. Mimosa pudica may be 

 mentioned as especially favourable for examination in this 

 particular. When this plant is removed from light to 

 darkness its leaflets droop, and the opposite pairs become 

 closely approximated to one another, so that their upper 

 surfaces are in contact. On being restored to light they 

 separate again and attain their former expanded condition, 

 but little time intervening before the change of position is 

 assumed in either case. Another very good instance is 

 afforded by Desmodium gyrans, the so-called Telegraph 

 plant, the rhythmic movements of whose lateral leaflets 

 have already been spoken of. During the day its leaves 

 are extended almost at right angles to the stem (fig. 155, A) ; 



