404 RELATION TO ENVIRONMENT. 



V. Leaf Patterns. 



MOSAICS, OB CLOSE PATTERNS. 



774. Where the leaves of a plant, or a portion of a plant, are 

 approximate and arranged in the form of a pattern, the leaves 

 fitting together to form a more or less even and continuous sur- 

 face, such patterns are sometimes termed "mosaics," since the 

 relation of leaves to one another is roughly like the relation of 

 the pieces of a mosaic. A good illustration of a mosaic is pre- 

 sented by a greenhouse plant Fittonia (fig. 441). The stems 



Fig. 441. 



Fittonia showing leaves arranged to form compact mosaic. The netted vena- 

 tion of the leaf is very distinctly shown in this plant. (Photo by the Author.) 



are prostrate and the erect branches quite short, but it may 

 have quite a wide system by the spreading of the runners; the 

 branches of such a length that the leaves borne near the tips all 

 fit together forming a broad surface of leaves so closely fitted 

 together often that the stems cannot be seen. The advantage 

 of a mosaic over a separate disposition of leaves at somewhat 

 different levels is that the leaves do not shade one another. Were 

 all the light rays coming down at right angles to the leaves, there 

 would not be any shading of the lower ones, but the oblique 

 rays of light would be cut off from many of the leaves. In the 

 case of a mosaic all the rays of light play upon all the leaves. 

 Some of the mosaics which can be observed are as- follows: 



