STIMULATION AND ITS RESULTS 371 
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 
behaviour 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 Leguminosae, the Oxalidacea, 
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. 153, a) ; 
as night draws on, the terminal leaflets droop till they 
assume a position almost or quite parallel to the stem 
