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. loi.) 



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). 



