264 SHADE PLANTS OF THE ALPINE FORESTS 



The leaves of the Wood Sorrel are of great 

 interest botanically, for they possess the power of 

 movement a rare occurrence among the Higher 

 Plants. At night, or in very strong sunlight, each of 

 the three leaflets falls, so as to hang with the apex 

 pointing directly downwards, and thus to lie parallel 

 with the leaf-stalk (Text-fig. XXV., 2). Under 

 ordinary circumstances, in daylight the leaflets are 



1. 2. 



FIG. XXV. The Leaves of the Wood Sorrel (Oxalis acetosella, Linn.). 

 1. In the day position. 2. In the sleep position. 



held at right angles to the stalk (Text-fig. XXV., 1). 

 This means that they receive the maximum of 

 illumination, whereas in very strong sunlight, by 

 assuming the second, or sleep position as it is called, 

 the light only falls very obliquely on the delicate 

 leaflets. At night, the sleep position protects the 

 leaflets from excessive loss of heat by radiation. 

 The mechanism of movement is seated in the very 

 short stalks of the leaflets, and the changes in 



