318 MODIFIED CIEOUMNUTATION. Chap. VU 



zenith and to free radiation as when the blade is 

 horizontal. Nevertheless, in a few instances, leaves 

 which seem to be prevented by their structure from 

 moving to so great an extent as 60° above or beneath 

 the horizon, have been included amongst sleeping 

 plants. 



It should be premised that the nyctitropic move- 

 ments of leaves are easily aifected by the conditions 

 to which the plants have been subjected. If the ground 

 is kept too dry, the movements are much delayed 

 or fail : according to Dassen,* even if the air is 

 very dry the leaves of Impatiens and Malva are 

 rendered motionless. Carl Kraus has also lately 

 insisted t on the great influence which the quantity of 

 water absorbed has on the periodic movements of 

 leaves ; and he believes that this cause chiefly deter- 

 mines the variable amount of sinking of the leaves of 

 Polygonum convolvulus at night ; and if so, their move- 

 ments are not in our sense strictly nyctitropic. Plants 

 in order to sleep must have been exposed to a proper 

 temperature : Erythrina crista-galli, out of doors and 

 nailed against a wall, seemed in fairly good health, 

 but the leaflets did not sleep, whilst those on another 

 plant kept in a warm greenhouse were all vertically de- 

 pendent at night. In a kitchen-garden the leaflets of 

 Phaseolus vulgaris did not sleep during the early part 

 of the summer. Ch. Royer saysjj referring I suppose 

 to the native plants in France, that they do not sleep 

 when the temperature is below 5° C. or 41° F. In 

 the case of several sleeping plants, viz., species of 



* Dassen, ' Tijdsehrift vor. Na- Bot.' (5tli series), ix. 1868, p. 345. 



lurlijke Gesch. en Physiologie,' t ' Beitrage zur Kentniss der 



1837, vol. iv. p. 106. See also Bew( gungen,' &c., in 'Flora,' 



Oh. Koyev on the importance of a 1879, pp. 42, 4.^, 67, &o. 



proper state of turgesramce of the % ' Annal. dea So. Nat. Bot.' 



cells, in 'Annal. def So. Nat. (5th Series), ix. 1868 p.366. 



