170 The Phenomena of Morphogenesis 



DORSIVENTRAL SYMMETRY 



In this type there is only one plane of symmetry, which extends 

 vertically through one dimension of the structure. The two sides are 

 alike but the front and back (or top and bottom) are not, thus distin- 

 guishing it from bilateral symmetry. It is characteristic of structures grow- 

 ing under an environment which is asymmetrical, as in the case of hori- 

 zontal ones, of those exposed to light on one side only, and of those 

 growing attached to some substratum. Among plants, creeping stems, 

 rhizomes, most leaves, many thalli, a wide variety of flowers, and, in 

 general, those structures which are not vertically oriented often show 

 dorsiventral symmetry. 



Dorsiventrality in plants is manifest in external form, in internal struc- 

 ture, and in physiological behavior. Single cells and coenocytes may 

 show such symmetry. Dorsiventrality may be genetically determined and 

 thus appear under various environments, or it may be directly induced 

 by environmental factors. Thus a dorsiventral structure may sometimes 

 become radial, and vice versa. In some cases the plant body may actu- 

 ally alternate between radial and dorsiventral symmetry, as in Mnium 

 undulatum and Cladonia verticillaris. 



Cases of dorsiventrality which are most obvious and easy to study are 

 those in structures that are typically horizontal, either because they are 

 weak and rest on the ground or because they are plagiotropic and tend 

 to grow in a horizontal position. 



In Thalli. Among lower plants many thalli are dorsiventral. The 

 coenocytic plant body of the alga Caulerpa is typically horizontal and 

 on its lower surface bears "rootlets" and on. its upper surface, "leaves." 

 The familiar heart-shaped prothallus of a fern is similarly dorsiventral, 

 bearing sex organs and rhizoids on its lower surface only. This type of 

 symmetry is characteristic of the plant body of many liverworts, both of 

 the thalloid and the leafy types ( Halbsguth, 1953 ) . The factors which in- 

 duce it in such plants have been studied by various workers (Fitting, 

 1935, 1950; Pfeffer, 1871; Bussmann, 1939; and others, p. 355). Fitting 

 studied especially the gemmae of liverworts. These are roundish, notched 

 plates of cells the dorsiventral orientation of which is determined by the 

 balance between light, gravity, and stimuli from the substrate, acting on 

 preformed meristems in the notch. Fern prothallia exposed to an en- 

 vironment without gradients ( shaken or on a turntable, p. 137 ) lose their 

 symmetry as well as their polarity. 



In Roots. Dorsiventrality is much less evident in roots than in stems. 

 Indeed, horizontally growing subterranean roots show little or no 

 change from radial symmetry either externally or internally. A few forms, 



