Observations on the Structure and Function of Hydathodes in Blechnum lehmannii

The fronds of many ferns in the Polypodiaceae (Ogura, 1972) and Cyatheaceae (Weiler, cited in Lippmann, 1925) possess swollen vein endings associated with specialized adaxial epidermal cells. Their structure is similar in all ferns (Gardiner, 1883; Potonie, 1892; Poirault, 1893; Goebel, 1930; Guttenburg, 1934), including Blechnum lehmannii Hieron. (Figs. 1-3). The depressed epidermis (Fig. 3) suggested the term "Wassergriibchen" for the vein endings to European anatomists of the nineteenth century (e.g., Potonie, 1892). Vein endings on developing fronds secrete water (Fig. 1) when conditions reduce or stop transpiration (Mettenius, 1856; Haberlandt, 1894a; Goebel, 1926, 1930). In some species, the secreted water contains salts in solution which form "chalk scales" after the water has evaporated (Mettenius, 1856; Potonie, 1892; Lippmann, 1925). Most sources report that water secretion is dependent on the metabolic activity of the specialized epidermal cells adaxial to the vein ending (Fig. 3, e); it is thus regarded as an active, or glandular, process (Gardiner, 1883; Haberlandt, 1894a, 1914; Lepeshkin, 1906; Stocking, 1956; Fahn, 1979). This conclusion is reflected in various references to the vein endings as water glands, salt glands, or chalk glands. There is, however, very little evidence to support any conclusions on the mechanism of secretion. All work on the subject was performed in the last century, and the results obtained are questionable due to the bias in experimental design which prevented testing of alternative hypotheses and to the possibility of artifacts in technique (see comments in Spanjer, 1898; Haberlandt, 1898; Lepeshkin, 1923). Moreover, the results are contradictory, since they have been interpreted as supporting either an active (Gardiner, 1883; Haberlandt, 1894a, 1914) or a passive (Spanjer, 1898) mechanism of secretion. Finally, all experiments were performed on a single species, Polypodium aureum L., and need not represent the situation in other ferns. Because the mechanism of secretion from vein terminations is unknown, they will be referred to in this paper by the term "hydathode" which, as originally defined by Haberlandt (1894b), did not presuppose a mechanism of secretion. Haberlandt used his term to refer to structures on above-ground portions of the plant, particularly foliage leaves, which function in water transfer to and from the plant surface. Previous work on the mechanism of hydathode secretion in ferns neglected important aspects of the problem. Despite the fact that root pressure could play a significant role in the secretion process, the relationship of root pressure to secretion in intact plants was not studied. In fact, there is no well documented account of root pressure in ferns. In addition, investigators could not analyze secretion from a structural standpoint because anatomical studies were confined to the mature, non-secreting hydathodes; the structure of the secreting hydathodes on developing leaves was unknown. In this study, the relationship of root pressure to secretion as well as developmental changes in hydathode structure were investigated in B.