LECTURE XXXVI. 



THE PERIODIC MOVEMENTS OF LEAVES AND FLORAL 

 ENVELOPES (SLEEP-MOVEMENTS). 



On glancing at the general aspect of the plants in a garden or a meadow 

 shortly after sun-down in the summer, it is obvious that the leaves of many of the 

 plants have assumed attitudes and positions different from those assumed in the 

 daytime. This is very striking, for example, in the case of plants like the Clovers, 

 and in all trees and shrubs {Robinia, Colutca, &c.) with similarly organised compound 

 leaves, especially where three or more leaflets are attached to a common leaf-stalk. 

 It is then found that the leaf-stalks are more erect or more pendent at night than 

 during the day, and the leaflets which they bear are drawn closely together, whereas 

 during the day they are usually extended horizontally. Similarly also with other 

 compound leaves, particularly those of the Clovers, &c., and, generally, all those (of 

 Cryptogams as well as Phanerogams) in which the laminae of the leaves are con- 

 nected with the shoot-axis or leaf-stalk by means of a special cylindrical peculiarly 

 organised portion. It scarcely needs mention that such plants cultivated in pots 

 and standing at the window or in a green-house, also exhibit the same phenomena. 



In these cases, then, in all Leguminosae and Oxalideae, and many less well- 

 known plants, it is always by means of peculiar organs at the base of the petiole, 

 or at the place where each lamina is connected with the stalk, that the changes 

 in position of the leaves are effected. Beside^ these organs, which put in motion the 

 parts situated on them by means of their up and down curvatures, all other 

 parts of the leaves, the laminae as well as the proper petioles, are non-motile — 

 i. e. they are only passively directed upwards and downwards by the motile organs 

 mentioned. 



As a rule the change consists in the leaves which are extended flat during the day, 

 and which present their chlorophyll-surfaces perpendicularly to the incident light where 

 possible, becoming folded together at night. With the commencing light of morning 

 the motile organs become curved, so that the laminae of the leaves situated on them 

 again assume the above-named extended diurnal position, while in the evening an 

 opposite curvature of these organs produces the nocturnal position. This phe- 

 nomenon has been termed the waking and sleeping of leaves ^ 



* The older literature as to the periodic movements of leaves and flowers may be here passed 

 over, inasmuch as it is treated in detail and criticised fully in Pfeffer's works. I shall therefore 



