THPJ SHAPES OF LEAVES. 



Wd 



transversely the circumstances are aKke. It is needless to 

 give diagrams to illustrate this extremely familiar truth. 

 Whether they are broad or long, oval or heart-shaped, pointed 

 or obtuse, the leaves of most trees and plants will.be remem- 

 bered by all as having the ends by which they are attached 

 unlike the free ends, while the two sides are alike. And it will 

 also be remembered that these equalities and inequalities of 

 development correspond with the equalities and inequalities 

 in the incidence of forces. 



§ 230. A confirmation that is interesting and important, 

 is furnished by the cases in which leaves present unsymme- 

 trical forms in positions where their parts are unsymmetri- 

 cally related to the environment. A considerable deviation 

 from bilateral symmetry may be seen in a leaf which habitu- 

 ally so carries itself, that the half on the one side of the midrib 

 is more shaded than the other half. The drooping branches of 

 the Lime, exemplified in Fig. 222, show us leaves so arranged 



and so modified. On examining their attitudes and their 

 relations one to another, it Tvill be found that each leaf is so 

 incKned that the half of it next the shoot grows oyer the 

 shoot and gets plenty of light ; while the other half so hangs 

 down that it comes a good deal into the shade of the pre- 

 ceding leaf. The result is that having leaves which fall into 

 these positions, the species profits by a large development of 

 the exposed halves ; and by survival of the fittest acting 

 along with the direct efiect of extra exposure, this modifi- 

 cation becomes established. How unquestionable is the 

 connexion between the relative positions of the halves and 

 their relative developments, will be admitted on observing a 

 42 



