Symmetry 171 



such as Isoetes, have roots that are not radial. In a number of orchids, 

 the air roots are dorsiventral in symmetry ( Janczewski, 1885; Goebel, 

 1915). This is especially conspicuous where the root is in contact with 

 a substrate (Bloch, 1935a). 



In Shoots. Horizontally growing stems and branches are often con- 

 spicuously dorsiventral. Notable examples of this are found among the 

 conifers where the lateral shoots tend to branch in a single plane and 

 thus form flat sprays (amphitrophy). This form may become so firmly 

 fixed that it persists even in cuttings (p. 189). In some species of Ly co- 

 podium and especially Selaginella, these flattened branch systems look 

 almost like much dissected compound leaves. Indeed, there is evidence 

 that the large pinnately compound leaves of ferns may have evolved 

 from such branch systems. In many horizontal shoots the leaves are 

 usually horizontal in orientation and confined to the two sides. This may 

 result from a torsion of the petioles which are actually inserted on the 

 stem in a spiral or decussate fashion or, more rarely, from an actual 

 modification of the phyllotaxy. 



Aside from this tendency to form flattened systems of leaves and 

 branches, the dorsiventral character of shoots is also conspicuous in the 

 dissimilarity of the leaves borne on the two sides. Such differences, to 

 which Wiesner gave the term anisophylly (1895), are common in 

 many plants and have been much discussed (Figdor, 1909; Goebel, 

 1928). 



Anisophylly is often induced by external factors, notably gravity and 

 light. It is particularly conspicuous in woody plants with opposite, decus- 

 sate leaves. In horizontal shoots of maple, for example, the upper mem- 

 ber of a vertically oriented pair is much smaller than the lower; and in 

 a horizontally oriented pair the upper half is smaller than the lower 

 (Fig. 7-10). Experiment shows that in many cases if shoots which would 

 normally be vertical are held in a horizontal position as they grow 

 from winter buds they become anisophyllous. In horizontal branches 

 twisted through 180° before their buds open, the new shoots show 

 reversed anisophylly, the lower leaves (originally on the upper side) 

 now becoming the larger. 



Anisophylly of this sort is present in certain species of Lycopodium 

 (such as the common ground pine, L. complanatum), where the creep- 

 ing rootstock is radially symmetrical but the ultimate branches flattened 

 and dorsiventral (though they are radial if grown in darkness, Fig. 7-11). 

 These branches have four rows of leaves, one on the upper side, one 

 much smaller on the lower, and two lateral ones, the lateral leaves being 

 the largest. Transitions from radial to dorsiventral symmetry are com- 

 mon, and the differences between the two are clearly due to environ- 

 mental factors. In conifers such as Thuja the ultimate branches are 



