. NYCTITROPIC MOVEMENTS 489 



influence on the behaviour of its leaves. This is best seen 

 in those plants whose leaves assume different positions during 

 the day and the night. The sensitiveness to the alternation 

 of light and darkness is not confined to ordinary foliage leaves, 

 but in many cases is shared by cotyledons also. The degree 

 of sensitiveness varies greath' in different plants. 



This form of irritabilitj' is manifested in a ver^- great degree 

 by manj^ of the Leguminosae. Mimosa pudica is perhaps the 

 most noteworthy of them all. When this plant is removed fi'om 

 light to darkness its leaflets close ; on being restored to light 

 they open again, 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 tele- 

 graph plant. During the day its leaves are extended almost 

 at right angles to the stem {fig. 1192 a) ; as night draws on the 

 terminal leaflets droop until they assume a position almost or 

 quite parallel to the stem [fig. 1192 b). Many others assume still 

 more curious positions, in some cases becoming twisted on their 

 petioles or folded together in various ways. In some, as in 

 Nicotiana glauca {fig. 1193), they rise instead of falling, and 

 become somewhat closely approximated to each other. 



These changes of position are generally spoken of as 

 nyctitropic or sleep movemonts, though the latter term is 

 misleading if it be interpreted to mean a sleep similar to that of 

 animals. It is not difficult to prove that these curious move- 

 ments are effected in response to the stimulus of the alternation 

 of light and darkness, or to a rhythmic difference in the amount 

 of light which they receive. AMien a plant which changes 

 the position of its leaves as described is placed for a time 

 in darkness, the periodic movement is soon ver^^ much 

 interfered with, and it ultimately stops. The cessation is not, 

 however, abrupt, but in most plants the movements will continue 

 for at least a day. Plants, again, which are found in other 

 countries to show this sensibility will when cultivated in England 

 perform the movements at the normal hours and not at times 

 corresponding to day and flight in the countries from which 

 they come. Nor is it the mere alternation of day and night ; it 

 is rather the difference between the illumination they receive 

 during the two periods that serves as the stimvilus, for some of 

 them will not assume the nocturnal position unless they have 

 been brilliantly illuminated during the day. The degree of sen- 

 sitiveness in this case is not so great as in those where the diurnal 

 and nocturnal positions are always regularly assumed. 



