VENTRAL STRUCTURES EFFECTING CAPILLARITY IN THE MARCHANTIALES

THE LIVERWORTS lack absorptive organs which penetrate deeply into the soil. They have neither special conductive tissues nor adequate protection against excessive water loss. Problems involving their water relations have long claimed the interest of bryologists, and some attention has been given to their external capillarity. There has, however, been' little specific observational or experimental work concerning the structural mechanisms involved in external conduction. Magdefrau (1937) reviewed the research in this field, and his summary clearly shows that further morphological analysis of structures related to water retention, conduction, and absorption is desirable. In the Marchantiales simplicity of external form is accompanied by a degree of structural specialization not found elsewhere among the hepatics. The increased evaporating surface afforded by the elaboration of dorsal areolae presents an additional problem of water conservation. Wenzl (1933) found that in Marchan.tia polymorpha the water transpired per hour, under extreme transpiration, equals the total water content of the thallus and that the loss is two to three times that of leaves of Betula pendula under the same conditions. For such liverworts the problem of sufficient water supply must be solved by external retention and ready absorption of water rather than by control of transpiration. Recently the writer (McConaha, 1939) analyzed the surfaces and spatial relations of the ventral appendages of Conocephalum conicum and showed their fundamental importance in the external conduction of water along the under side of the thallus. Careful survey of the size, form, number, and distribution of these ventral organs revealed a highly 1 Received for publication December 10, 1940. The writer wishes to express her sincere thanks to Professor R. B. Wylie for his helpful guidance during this investigation. specialized capillary system capable of rapid water movement in either direction along the under side of the thallus. External capillarity tends to compensate for the lack of internal conductive channels and to permit the growth of relatively large thalli. The writer has extended this study to several other members of the Marchantiales in an attempt to determine their variations in morphological specialization, the efficiency of their capillary systems, and the importance of external conduction. No attempt has been made to evaluate quantitatively environmental factors or to estimate surfaces involved in water loss. MATERIALS AND METHODS.-Preissia quadrata (Scop.) Nees, Reboulia hemisphaerica (L.) Raddi, Marchantia polymorpha L., and Lunularia cruciata (L.) Dumort. were selected for comparative morphological study. Thalli of Preissia, Reboulia, and Marchantia were collected from several natural habitats in eastern Iowa. Collections of Lunularia were made in a commercial greenhouse. In all instances typical thalli were selected in sufficient numbers to constitute a representative sampling of each species. Ventral surfaces of the thalli were studied directly with a biobjective binocular and rhizoidal counts were made under a magnification of 85 diameters. Drawings of ventral surfaces showing scale arrangement were made with the aid of a Leitz projector. Scale area and ventral thallus area were measured from these projection drawings, using a planimeter. Length and diameter of rhizoids were measured with an ocular micrometer and computations of rhizoidal area were made on the basis of their numbers on one centimeter of thallus length. These areas were translated, for comparative purposes, into ratios of rhizoidal surface to ventral thallus area (table 1).