422 RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



These cannot be of use as apertures, through which light can pass to leaves 

 situated below, for the simple reason that, as a rule, no other leaves requiring light 

 are to be found under the branches in question. Smaller green leaves are now 

 inserted in these gaps, which serve as protective leaves for the flowers, that is, 

 indirectly for the fruits, but whose function also coincides entirely with that of 

 the large foliage-leaves. The small leaves twist and turn until each comes to lie 

 exactly in the middle of a gap, where they neither encroach upon the large leaves, 

 nor are encroached upon by them. An exactly similar insertion of smaller leaves 

 in the gaps between the larger can also be observed in the Thorn-apple (Datura 



Fig. 112.— Mosaic of Unsymmetrical Leaves of unequal size. 

 Leafy horizontal Twig of an Elm {Ulmus) seen from above. 



Stramonium), and in Imjjatiens parvijlora, illustrated respectively in fig. 104 ^ 

 and fig. 104 \ This mosaic-like fitting together of larger and smaller blades 

 appears to be combined with the want of symmetry of the leaf-base in short- 

 stalked leaves, as e.g. in the wall-climbing stem of Ficus scandens (see fig. 110 2), 

 and on the older horizontal branches of elms {Ulmus), one of which is illustrated 

 in fig. 112. It has been already mentioned that the blades with erect petioles, 

 arranged in the central rows on the Paper Mulberry, are considerably smaller than 

 the lateral rows of leaves with horizontal stalks (see fig. 108). This difference in 

 the size of the central and lateral rows of leaves on horizontal stems is very 

 noticeable also in the dainty selaginellas, belonging to the family of Lycopodiaceae, 

 of which a species (Selaginella Helvetica) is represented in fig. Ill \ 



It is worth noticing that the occurrence of leaves of two sizes on the same stem, 

 as well as the mosaic-like arrangement and fitting together of the leaves in one 



