LEAVES, THEIR FORM AND MOVEMENT 



Si 



136. Relation of leaves to light. This is preeminently a 

 subject for field or outdoor study. Observations show that leaves 

 assume the most advantageous arrangement and position to receive 

 the best lighting. In many cases, when the leaf arrangement on 

 the stem may be three, five or eight ranked, the leaf blades may be 

 all arranged in a single plane, to receive the light from one direction. 

 This often occurs in woods or groves, the petioles of the leaves 

 twisting so as to allow the blade the most favorable position. 

 Mosaics or patterns are formed where a number of leaves on a 

 single shoot lie so that they are fitted in almost like pieces of mosaic 

 and so that there is very little shading of adjacent leaves. Fit- 

 tonia grown in greenhouses is a splendid example. Rosettes are 

 formed when the leaves are crowded on the stem near the ground 

 in the form of a rosette. Imbricate patterns are seen where the 

 leaves are not so closely crowded, but overlap something like 

 shingles on a roof so that light can reach the leaves. In the radiate 

 pattern the leaves radiate in all directions from horizontal to the 

 vertical and thus obtain a good light relation, as in the screw pine 

 (Pandanus) often grown as an 



ornamental plant. 



137. Irritability of ten- 

 drils and twining stems. 

 When a tendril or a twining 

 stem, as it slowly swings 

 around, comes in touch or 

 contact with some object, this 

 contact stimulus causes it to 

 bend at this point bringing 

 new points in contact so that 



trap 



the tendril Or Stem then Coils P ul . a )- showing winged glandular hairs folding 



petiole and toothed inward as a result of a 



lo 



Fig. 71. 



Leaf of Venus fly- 

 trap (Dionaea 



Fig. 72 



Leaf of Drosera ro- 

 tundifolia, some of the 



lobes - 



stimulus. 



around the object of support. 



138. Response of insectivorous plants to touch. Remark- 

 able movements are shown by the leaves of some insectivorous 

 plants. In the Venus's flytrap (fig. 71) the terminal part of the 

 leaf is shaped like a steel trap. The blade is broad and the margin 

 rounded and beset with numerous hairs or spines resembling the 



