ASYMMETRY OF LEAVES 115 



alone peltate, and in proportion as the shoot-axis elongates in preparation 

 for the formation of an inflorescence the leaves become shorter-stalked, 

 the peltate form disappears, and the ordinary form of leaf, with a lamina 

 continued out of the stalk, is developed l . The lamina of leaves of this 

 kind is usually so constructed that its median plane divides it into two 

 nearly equal sides. In orthotropous shoots this median plane which 

 falls in the midrib and stands at right angles to the surface of the leaf 

 is one vertical to the horizon. The symmetry of the sides of the leaf 

 is however in reality only approximate. 



Neither in the configuration of the outline nor in the arrangement 

 of the venation of the leaf is complete symmetry anywhere met with. 

 The development of the leaf of a fern or of a moss, for example, shows 

 from the outset an unequal construction of the two sides of the leaf 2 . 

 Similarly in the leaf of a dicotyledonous plant with feathered venation, the 

 corresponding nerves on the two sides of the leaf are only seldom opposite 

 one another. Apart from this general absence of complete symmetry 

 there are some cases which require special notice, in which the asymmetry 

 of the leaf is extremely marked, and is a constant character of the plants 

 in question 3 . Asymmetry occurs most abundantly in foliage-leaves, and 

 in them can almost always be brought into connexion with their biological 

 relationships and especially with their position, using this word in the 

 widest sense ; but asymmetry is frequent also in cotyledons, prophylls, 

 and bracts. In some plants the cotyledons only are asymmetric, the 

 other leaves are not so, for instance in species of Geranium and Poly- 

 gonum ; whilst in others only the foliage-leaves and not the cotyledons 

 show asymmetry. The following are some examples : 



i. COTYLEDONS. 



Examples of asymmetry in these are found in Geranium pratense and 

 other species, Erodium, Lupinus, Astragalus, Cicer, Tetragonolobus, 

 Desmodium gyrans, Polygonum Fagopyrum, and others. We may 

 acquiesce with Lubbock 4 in regarding the inequality in the sides of 

 the cotyledonary leaves as a consequence of their position in the seed 



1 Quite other relationships obtain in the shorter-stalked peltate leaves of some epiphytic Hymeno- 

 phyllaceae. For an account of these see the pages upon *he configuration of the leaves of ferns in 

 Part II of this book. 



2 See what is said on this subject in Part II of this book. 



3 See the account by Wydler, Uber asymmetrische Blatter und ihre Beziehung zur Symmetric der 

 Pflanzen, in Flora, 1857, p. 209; Herbert Spencer, Principles of Biology, ii. chap. ix. ; Wiesner, 

 Untersuchungen iiber den Einfluss der Lage auf die Gestalt der Pflanzenorgane : I. Die Anisomorphie 

 der Pflanze, in Sitzungsber. d. Akad. Wiener, 1892, and other communications of the author cited 

 there. 



4 Lubbock, A contribution to our knowledge of seedlings, vol. i. p. 34. London, 1892. 



I 2 



