ASYMMETRY OF LEAVES 



117 



I have already pointed out that in some Commelinaceae the chief 

 shoot is radial, the lateral shoots are dorsiventral. In Callisia delicatula 

 the radial chief shoots have a phyllotaxy and symmetric leaves, the 

 lateral shoots have alternating asymmetric leaves. The leaves of the 

 plagiotropous twigs of the lime are with few exceptions more or less 

 asymmetric, but in cuttings placed erect in the soil Herbert Spencer 

 found a considerable percentage of the leaves quite symmetric. In the 

 elm and the beech the proportion of asymmetry in the leaves varies, but 

 radial shoots of seedlings have always symmetric leaves. On the 

 plagiotropous shoots the leaf which stands at the end of the shoot always 

 has its median plane at right angles to the horizontal, and is not infre- 

 quently almost symmetric 1 , inasmuch as both sides of the lamina extend 

 nearly equally far down the petiole, whilst in the asymmetric leaves the 

 lower side (directed to the base of the shoot) extends further down the 

 petiole than does the higher. 



The asymmetry is then in these 

 cases not strongly inherited, and can 

 be changed by external influences. 

 I do not doubt that it exists in all 

 leaves primarily in a small degree, and 

 is increased in many cases through 

 external influences of which light and 

 not gravity is the chief. The following 

 instance, which I mention here al- 

 though it rightly belongs to another 

 section of this book, may be taken as an 

 illustration of how, in many cases, light 

 can influence directly the form of a leaf. 



In Trichomanes Hildebrandti 2 the leaves stand apparently in one 

 line upon the back of the rhizome which creeps on the surface of the 

 stems of trees ; they are sessile and peltate, and their under sides bear 

 hair-roots by which they cling closely to the substratum. Wherever 

 one leaf is overlapped by another its growth soon ceases, and the further 

 development of the overlapping leaf is at the points of overlap retarded, 

 and in this way irregularities in the development arise. Observation 

 of the living plants can alone determine with certainty the influential 

 factors here, but it is fair to assume that in the overlapped leaf shading 

 arrests the growth of the leaf. Whether the arrest of growth in the over- 

 lapping leaf is to be ascribed, as Giesenhagen has it, to the fact of its 



FIG. 6S. Kochea falcata. Transverse section 

 through the terminal bud of a seedling. The leaf- 

 pairs are obliquely crossed and the leaves are 

 asymmetric. 



1 See Wiesner in the papers cited in the note on p. 115. Wiesner, founding on a remark of 

 Hofmeister, assumes that the symmetric leaf at the end of the shoot has arisen from an asymmetric 

 primordium, but in the absence of developmental proof I c 1 ibt this. 



2 See Giesenhagen in Flora, 1890, p. 450. 



