IRRITABILITY. 539 



position so long as the sunlight continues to fall upon them ; 

 their position being quite independent of the direction of 

 the incident rays. This matter has not been fully investi- 

 gated, but, as Pfeffer points out, it suggests that there is a 

 certain optimum intensity of light under which the leaves are 

 fully expanded, and that any increase beyond this acts like a 

 diminution in causing them to close. 



With regard to the particular rays of the spectrum which 

 determine these movements, it appears, from the observations 

 of Sachs and of Bert, that the highly refrangible rays are 

 those which are especially concerned. When a plant with 

 its leaves fully expanded, is exposed to yellow light, the leaves 

 soon close ; whereas if, under the same circumstances, the 

 plant be exposed to blue light its leaves will remain expanded. 

 Yellow light acts like darkness : blue light like daylight. 



In consequence of their sensitiveness to variations in the 

 intensity of light, motile leaves perform periodic movements 

 in accordance with the normal variations which take place 

 within the twenty-four hours. When darkness comes on they 

 close, and when it again becomes light they unfold. The move- 

 ment of closing is termed the "sleep" or nyctitropic movement, 

 and recalls those exhibited by some growing floral and foliage 

 leaves. The closed condition is what is known as the nocturnal 

 position, the expanded, as the diurnal position. The normal 

 alternation of day and night thus impresses on these motile 

 organs a daily periodicity of movement, just as it impresses 

 on growing organs a daily periodicity of growth (p. 400). 



Periodic movements occur generally in the orders Leguminosae and 

 Oxalidaceae, and they have been observed in various plants belonging to 

 other orders ; in Phyllanthus Niruri (Euphorbiaceae), Porlieria hygro- 

 metrica, (Zygophyllaceae), among Dicotyledons ; in Thalia dealbata and 

 Maranta arundinacea (Cannaceae), Strephium floribundum (Gramineae), 

 among Monocotyledons ; and in Marsilea qiiadrifolia among Vascular 

 Cryptogams. (See Darwin.) 



The sleep-movement has in all cases this result, that the 

 upper surface of the leaf, instead of being horizontal, is 

 brought more or less nearly into a vertical plane, but the 

 direction of the movement is different in different cases. 



