362 VEGETABLE PHYSIOLOGY 



This rhythmic change in the protoplasm is not exhibited 

 by organs during growth only ; in many cases it persists 

 throughout their life. Very conspicuous instances of it are 

 afforded by certain movements often exhibited by the leaves 

 of particular plants. Perhaps the most familiar of these is 

 the so-called Telegraph plant, Desmodium or Hedysarum 

 gyrans. Its leaves are ternate, the terminal leaflet a being 

 very large in comparison with the two lateral ones b (fig. 151). 

 If the plant is watched while exposed to suitable tempera- 

 ture and illumination, the lateral leaflets are found to move 

 up and down on the rachis, sometimes passing through an 



angle of 180, and twisting slightly 

 as they move. They thus describe 

 a kind of ellipse, the duration of 

 the movement being about two 

 minutes. Many other instances of 

 a similar kind are known, the 

 Leguminoscp furnishing many ex- 

 amples. All of them do not exhibit 

 the movements with the same ease, 

 as they are interfered with by 

 other changes in position which 

 FIG. 161. TERNATE LEAF OF result from external stimulation. 



THE TELEGRAPH PLANT (Des- mi ,, , , .-1,1 



medium gyrans). They can often be made evident by 



keeping the plant under constant 



external conditions. Darkness, however, if too prolonged 

 causes their cessation, though in some cases they are made 

 evident by deprivation of light for a short time. The 

 mechanism of the movement in most of these cases is 

 the rhythmically varying turgescence of particular organs 

 known as pulvini, which are situated at the bases of the 

 stalks of the leaves or leaflets. As in the cases already 

 noticed, the alternations in the turgescence are the expres- 

 sion of rhythmic changes in the protoplasm of the cells of 

 each pulvinus. As these pulvini play a considerable part in 

 the changes of position which are exhibited by many leaves 

 under various conditions, their structure may well attract 



