88 GROWTH AND WORK OF PLANTS 



similar cushions at the base of the leaflets and pinnae. At night, 

 or when placed in darkness, the leaflets close up, and the leaves 

 droop, opening up again with the coming of light. During the 

 day if the sunlight is too strong, the leaves adjust themselves to 

 the profile position, i.e., with the edge toward the source of strong- 

 est light. When the soil becomes too dry and the plant is in 

 danger from loss of water, some or all of the leaflets close and 

 the leaf droops, regulating itself according to the degree of dryness 

 or drought, since in the closed position the leaves lose water less 

 rapidly. 



140. Leaf patterns or mosaics. This is preeminently a 

 subject for field or outdoor study. The position of the leaves 

 becomes fixed. In many cases, when the leaf arrangement on 

 the stem may be three, five or eight ranked (paragraph 133) 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. Fittonia 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 direc- 

 tions from horizontal to the vertical and thus obtain a good light 

 relation, as in the screw pine (Pandanus) often grown as an 

 ornamental plant. 



