ADJUSTMENT TO SURROUNDINGS 
141 
as to bring the leaf -blade into poor light-exposure, the 
leaf-stalk, in some plants, will bend and twist so as to 
restore the blade to a suitable light-relation. 
The leaves of some plants (e.g., nasturtium) remain 
adjustable to the direction of incident light during their 
active life (Fig. 95) ; the leaflets of legumes and some other 
FIG. 100. Vertically growing branch of a maple, side view, showing 
elongation of the petioles of the lower leaves, which serves to prevent their 
being shaded too much by the leaf-blades above them. (Cf. Fig. 101.) 
plants (Fig. 96) fold together each night and on cloudy 
days, thus manifesting the so-called "sleep" of plants. 
Whether this response is of value to the plant is not 
entirely certain. In other plants (e.g., the house geranium) 
the leaves have a fixed light-position, and after reaching 
maturity are not able to readjust themselves to changed 
conditions of illumination (Figs. 97 and 98). 
