DOR'SIVENTRAIIITY (from Lat. dorsum, back -I- renter, In plants. a term of sym metry. indicating that a structure has two dis simila• (dorsal and ventral) faces, which are dif ferently related to the environment. For ex ample, the thallus body of an ordinary liverwort, like Alarehantia, is dorsiventral. It is prostrate, so that one surface (the dorsal) is exposed to the air and sunlight. and the other (the ventral) is in contact with the earth or other substance on which the plant grows. As a consequence of these different exposures. the two surfaces arc not only unlike iu appearance, but they give rise to different organs. In Alarchantia. for example, the dorsal tissues, heing exposed to light, de velop chlorophyll, and the dorsal surface, being free, gives rise to the reproductive bodies: while the ventral surface puts out rhizoids (simple root like structures) which penetrate the substratum and act as holdfasts and absorbing organs. Ordi nary horizontal leaves are illustrations of dorsi ventral organs, the two surfaces usually differ ing in appearance, and the dorsal and ventral tis sues differing even more decidedly in their ar rangement. For example, the dorsal (upper) epidermis of such a leaf is often made up of compact cells without stomata ( *brea thing pores'), and is covered by a well-developed cuti cle: while the ventral (lower) surface contains multitudes of stomata and may be hairy. In
such a leaf the dorsal working cells (inesophyll) are elongated and packed closely together to form 'palisade tissue,' while the ventral cells are loosely aggregated. These differences in struc ture are due to the character of the exposure, as is evidenced by the fact that if a very young dorsiventral leaf is fixed in an inverted position, the normally dorsal region will develop the ven tral arrangement of tissues, and vice versa. Many underground stems, such as those of the common ferns and the ordinary rootstocks of seed plants, are dorsiventral, giving rise to roots be neath and leaves or branches above. As con trasted with a dorsiventral structure. the 'radial structure' is one in which all sides are exposed alike. and such organs as are produced are repeat ed uniformly about the centre, as in an ordinary upright stem with its radially arranged leaves. See LEAF.