146 



BOTANY. 



191. Leaves are for the most part bilaterally symmetrical, 

 a vertical plane passing from base to apex generally dividing 

 them into two equal and corresponding halves. In the elm, 

 linden, begonia, etc., and the leaflets of many compound 

 leaves, the two halves are unequal. The asymmetry is ap- 

 parently related in some way to the position of the leaves on 

 the stem, as it is more frequently noticed on plants whose 

 leaves are two-ranked, with the leaf planes parallel, or 

 nearly so, to the axis of the stem (or in compound leaves, to 

 the central leaf axis). In some two-ranked leaves the upper 

 half of each leaf (i.e., that nearer to the apex of the stem) 

 is the larger, while in others the opposite is the case.* 



192. In form leaves are very 

 variable ; even in the same plant 

 it rarely happens that all have 

 the same form. In general, 

 elongated forms (i.e., linear and 

 oblong) prevail in the Monocoty- 

 ledons, while as a rule they are 

 considerably broadened (i.e., 

 lanceolate, elliptical, cordate, 

 etc.) in mosses, ferns, and Di- 

 cotyledons ; many exceptions, 

 however, occur. 



' 193. The absolute size of 

 leaves varies greatly also. The 

 largest leaves as, for example, those of palms, tree-ferns, ba- 

 nana, Victoria regia, etc. occur in the warmer portions of 

 the earth ; in frigid regions the leaves are small ; in tem- 

 perate climates perennial leaves are, as a rule, smaller than 

 annual ones. 



* See an article on this subject by Professor Beal in American 

 Naturalist, 1871, p. 571, and a still earlier one by Dr. Wilder. Both 

 writers show that in many cases the upper half of the leaf is the most 

 developed, in opposition to De Candolle, who makes the statement 

 that " the side most developed is always the lower." Herbert Spencer's 

 supposition that the want of symmetry is (in some cases) due to the 

 shading of the smaller half of the leaf, they show not to be correct, as 

 the asymmetry is observable in the voung leaves in the unexpanded 

 bud! 



B C 



Fig. 124. A, leaf with serrate mar 

 gin : , leaf with dentate or toothed 

 margin ; C, leaf with crenate or scal- 

 loped margin. 



