RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 423 



plane, is observed especially in plants growing in dark or half -shaded places. 

 There they do not require to protect themselves against an over-abundance of light, 

 but on the contrary have to make what use they can of its scanty amount, and 

 this is best effected by the fitting together of all the leaves on a stem in one plane, 

 like the stones of a mosaic. It is, of course, not so easy to produce a mosaic from 

 symmetrically circular or elliptical leaves; but unsymmetrical, or rhomboidal, 

 triangular, pentagonal, and, generally, polygonal blades lend themselves particularly 

 well to this arrangement. Excellent examples of this are furnished in the leaf- 

 mosaics in fig. 110, as well as in the elm twig represented opposite. The leaf -mosaic 

 formed by the ivy on the ground of shady woods is particularly instructive in 

 this respect. In the picture below, which is a faithful reproduction of a piece 

 of ivy carpeting the ground of a wood, it is seen how the lobed, five-pointed leaves 



Fig. 113— Leaf-mosaic. 

 Ivy on the ground of a forest. 



have in the course of time fitted into one another. The lobes and points of one fit 

 into the indentations of another, and thus originates a layer of leaves than which 

 one better fitted to the given external conditions could hardly be imagined. In this 

 mosaic, indeed, we no longer see two rows of leaves symmetrically arranged on the 

 horizontal stem. What manifold elevations and depressions, torsions, displace- 

 ments, and elongations must have taken place in order to produce such a leaf- 

 mosaic from the regular rows of leaves! But we learn from the consideration of all 

 these instances, that not only the arrangement and distribution of the foliage, and 

 the direction and length of the leaf-stalks, but the size and even the shape of 

 the leaf -blades also, and the resultant mosaic-like piecing together, stand in causal 

 relation to the conditions of illumination; and that in dimly-lighted situations 

 plants endeavour to utilize, and turn to account, the sunlight for the green tissue of 

 the foliage-leaves as far as possible by the means at their disposal, and with regard 

 to the given conditions of space. 



