b 



II GERANIACE^ 135 



acid flavour. They are very delicate, and adapt them- 

 selves to diflferences of light in four ways — by movements 

 of the whole leaf, by change of angle, by movements 

 of the chlorophyll within the leaf, and by changes in the 

 form of the chlorophyll grain. In strong sunshine the 

 leaflets close slightly and move downwards (Fig. 78), 

 so that the rays fall on them less directly ; and also at 

 night, serving as a protec- 

 tion against cold. Dar- 

 win's experiments clearly 

 showed that leaves which 

 were compelled to remain 

 horizontal at night suifered 

 more from frost than those 



which were allowed to Fig. IS.—OxaHs Acetusella. a, Leaf seen 



assume a vertical position.^ abo™trnight ^''^'' *' ^^^^ ''''' *^'™' 



The motile organs are 

 the very short stalks which connect the leaflets with 

 the common leaf- stalk. Fig. 79 represents a trans- 

 verse section in Oxalis carnea. Beneath the skin or 

 epidermis is a mass of parenchyma surrounding an axial 

 strand, containing the vascular tissue enclosed in a 

 sheath. Fig. 80 gives a longitudinal section. The par- 

 enchyma presents no important difl"erences. If the 

 lower part swells and the upper contracts, as in Fig. 78, 

 the leaflet rises ; if the reverse, it sinks. The change is 

 rendered possible by the extrusion or absorption of water. 



It is remarkable that if the leaves are covered up at 

 night so that they receive no light they will still open 

 in the. morning.^ The movements are mainly due to 

 the chemical rays. If the yellow, orange, and red rays 

 are excluded, the plant reacts as if in white light ; on 

 the contrary, if the violet and blue rays are shut out, 

 and the red end of the spectrum only allowed to "pass 

 through, the plant behaves as if in darkness. 



Under normal conditions the cell represents a turgid 

 bladder ; the watery cell-sap being imprisoned by the 



I Movements of Plants. 

 ^ Saclis, Lectures on Physiology of Plants. 



