318 MODIFIED CIECUMNUTATION. CHAP. VU 



zenith and to free radiation as when the blade ig 

 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 affected 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 f 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. Boyer says,J 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, ' Tijdschrift vor. Na- Bot.' (5th series \ ix. 18GS, p. 345. 



lurlijke Gcseh. en Physiologic,' f ' Beitriige zur Kentuiss der 



1837, vol. iv. p. 106. See also Bewt gungcn,' &c., in 'Flora,' 



Ch. Rover on the importance of a 1879, pp. 42, 43, G7, &c. 



proper state of turgesccnce of the J Annal. des Sc. Nat. Bot.' 



cells, in 'Aiinal. det Sc. Nat. (5th Series), ix. 1868 p.36G. 



