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. ts leaves are ternate, the terminal leaflet a being 
very large in comparison with the two lateral ones b (fig. 151). 
It 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 
Leguminose 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 
ey eae Ae result from external stimulation. 
modium 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 
