534 MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



is tlie diminutiou of light and heat after sunset, and since the rising and sinking of 

 the leaflets effected thereby coincides with the nocturnal sleep of birds and other 

 animals, the phenomenon described has been interpreted in this sense, and termed 

 the slcejj of plants. 



The rapidity with which the movement of the leaflets is accomplished varies 

 very much in different plants, and even in the same species is sometimes quicker, 

 sometimes slower, according to external influences. All the circumstances which 

 increase the turgidity of vegetable-cells have also an accelerating effect on these 

 movements. It is still an unsolved problem how far light and darkness influence 

 the turgidity of tlie pulvini. It is supposed that the darkening produces an 

 increased afflux of water and an increase of turgidity in the whole pulvinus, but 

 more rapidly in one half than in the other; while the protoplasm in one half of 

 the cells of the pulvinus is stimulated by light to give up a portion of the watery 

 sap, lying at the time within the sphere of its influence, to the surroundings — by 

 which indeed not very much is explained. 



In one section of plants whose leaflets assume a sleep position when darkness 

 sets in after sunset, the leaflets provided at the base with pulvini move upwards, 

 and in the other section downwards. The movement is upwards as a rule in 

 ternate leaves, of which the clover may serve as a type. When the elevation has 

 been accomplished, the leaflets are directed either all three almost at a right angle 

 to the horizon, or the terminal leaflet is bent up rather more than the two lateral 

 ones. A very pretty example of this is furnished by Tetragonolobus siliquosus, 

 represented in figs. 133 " and 133 *, and also by Desmodiwm penduliflorum as well 

 as by various species of Lotus, Trifolium, Melilotus, Medicago. Pinnate leaves, 

 whose leaflets rise up and arrange themselves next one another like the leaves of a 

 closed book, are found on numerous small scrubby mimosa bushes of Peru, of which 

 a species, viz. Mimosa Lindheimeri is represented in figs. 133 ^ and 133 -, in the 

 day and night positions. In the Australian Acacia lophantha and several other 

 true acacias, in SchranJcia aculeata and species of ^schynomene, in the American 

 gleditschias, further in the Australian Clianthus Bampieri and in the widespread 

 European Goronilla varia. In fig. 133^ is shown how the erect leaflets of the 

 Goronilla lie against one another very regularly in pairs. Just as often, instances 

 are observed in which the leaflets of the pinnate or digitate leaves fall downwards 

 after sunset. An example of this group is afforded by the leaf of one of numerous 

 American amorphas (Amorpha fruticosa), which is illustrated in figs. 133^ and 

 133 *. These leaflets which droop at night are also very noticeable in the Indian 

 Averrhoa Caramhola, in various species of indigo and liquorice (Indigofera and 

 Glycyrrhiza), in the sophoras {e.g. Sophora alopeeuroides), in the American tree, 

 Gymnocladtis Canadensis, and in robinias, of which Rohinia Pseudacacia (popu- 

 larly called acacia) is planted everj^where as a decorative tree. In like manner in 

 the widely-spread common Wood Sorrel (Oxalis Acetosella), cf. fig. 90*, in the 

 Indian pinnate-leaved Oxalis sensitiva, and in numerous American sorrels. 



With respect to the radiation, it is immaterial whether the leaflets rise up or 



