American Fern Society Observations on the Structure and Function of Hydathodes in Blechnum lehmannii Author(s): John S. Sperry Source: American Fern Journal, Vol. 73, No. 3 (Jul. - Sep., 1983), pp. 65-72 Published by: American Fern Society Stable URL: http://www.jstor.org/stable/1546852 Accessed: 11/09/2009 13:15 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=amfernsoc. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. American Fern Society is collaborating with JSTOR to digitize, preserve and extend access to American Fern Journal. http://www.jstor.org AMERICANFERNJOURNAL: VOLUME 73 NUMBER 3 (1983) 65 Observationson the Structureand Function of Hydathodes in Blechnum lehmannii JOHN S. SPERRY* The fronds of many ferns in the Polypodiaceae(Ogura, 1972) and Cyatheaceae (Weiler,cited in Lippmann, 1925) possess swollen vein endings associated with specializedadaxial epidermal cells. Their structureis similarin all ferns (Gardiner, 1883; Potonie, 1892; Poirault, 1893; Goebel, 1930; Guttenburg,1934), including Blechnum lehmannii Hieron. (Figs. 1-3). The depressed epidermis (Fig. 3) sug- gested the term "Wassergriibchen"for the vein endings to Europeananatomists of the nineteenthcentury (e.g., Potonie, 1892). Vein endings on developingfronds secrete water (Fig. 1) when conditions reduce or stop transpiration(Mettenius, 1856; Haberlandt,1894a; Goebel, 1926, 1930). In some species, the secretedwater containssalts in solution which form "chalkscales" after the waterhas evaporated (Mettenius,1856; Potonie, 1892; Lippmann,1925). Most sourcesreport that watersecretion is dependenton the metabolicactivity of the specialized epidermalcells adaxial to the vein ending (Fig. 3, e); it is thus regardedas an active, or glandular,process (Gardiner,1883; Haberlandt,1894a, 1914; Lepeshkin,1906; Stocking, 1956; Fahn, 1979). This conclusionis reflectedin variousreferences to the vein endings as waterglands, salt glands, or chalk glands. Thereis, however,very little evidenceto supportany conclusionson the mechanism of secretion. All work on the subject was performedin the last century,and the results obtained are questionabledue to the bias in experimentaldesign which preventedtesting of alternativehypotheses and to the possibility of artifactsin technique (see comments in Spanjer, 1898; Haberlandt,1898; Lepeshkin, 1923). Moreover,the results are contradictory,since they have been interpretedas support- ing eitheran active (Gardiner,1883; Haberlandt,1894a, 1914) or a passive(Spanjer, 1898) mechanismof secretion.Finally, all experimentswere performedon a single species, Polypodiumaureum L., and need not representthe situationin otherferns. Becausethe mechanismof secretionfrom vein terminationsis unknown,they will be referredto in this paperby the term"hydathode" which, as originallydefined by Haberlandt(1894b), did not presupposea mechanismof secretion.Haberlandt used his term to refer to structureson above-groundportions of the plant, particularly foliage leaves, which functionin watertransfer to and from the plant surface. Previous work on the mechanism of hydathodesecretion in ferns neglected importantaspects of the problem. Despite the fact that root pressurecould play a significantrole in the secretionprocess, the relationshipof root pressureto secretion in intactplants was not studied.In fact, thereis no well documentedaccount of root pressure in ferns. In addition, investigatorscould not analyze secretion from a structuralstandpoint because anatomical studies were confined to the mature, non-secretinghydathodes; the structureof the secretinghydathodes on developing leaves was unknown.In this study, the relationshipof root pressureto secretionas well as developmentalchanges in hydathodestructure were investigatedin B. * Harvard Forest, Petersham, MA 01366. Volume 73, number 2, of the JOURNAL was issued 16 June 1983. 66 AMERICANFERNJOURNAL: VOLUME 73 (1983) ?-?WtI.N..' ALSC FIG. 1. Hydathodessecreting water on a developingfrond; arrow indicates a dropletof waterabove a hydathode.FIG. 2. Epidermalpeel of hydathodephotographed with darkfield illumination.There is no intercellularspace in the epidermallayer; the separationof cells markedby the arrowis an artifactof preparationdue to the weak adherencebetween epidermalcells. FIG. 3. Transversesection of a hydathodeon a maturefrond made parallel to the leaf marginand treated with I-KI/H2S04.e = epidermal cells of hydathode,en = endodermis;arrows indicate Casparian strip. lehmannii in its native locality, the Central American cloud forest. Observations suggest that one mechanism of secretion from fern hydathodes is the passive transmission of xylem sap from vein endings to the plant surface through the apoplast of the hydathode under a pressure gradient induced by root pressure. MATERIALSAND METHODS Plants growing in the cloud forests near Xalapa in the state of Veracruz, Mexico; provided material for both experimental and anatomical work. Individual plants consist of a single, erect rhizome with short internodes. Root pressure was measured with a bubble manometer attached to the cut stipe (Fig. 4A) and a thermometer. The manometer was made with surgical tubing (ca. J. S. SPERRY:OBSERVATIONS ONBLECHNUM HYDATHODES 67 ) 1.20- S E ?-~ o S 1.10- E C 1.001 BB 0.5 1.0 2.0 A B time(hrs.) FG. 4A. Manometerattachment for measuringroot pressure.B= clearplastic bag enclosingthe entire crownof an individualplant, S =cut stipe to which the manometeris attached,C = clamp, Y = forked connector,BB = bubbleat sealed end of the manometertube. FIG. 4B. Rootpressure vs. time for plants 1, 2, 3. S = onset of hydathodesecretion. 0.8 mm inside diameter) partially filled with acid fuchsin and sealed at one end so as to leave a bubble at the sealed end (Fig. 4A, BB). Bubble length was assumed to be proportional to bubble volume. At the beginning of the experiment, bubble length at ambient pressure and air temperature was determined by releasing the clamp (Fig. 4A, C). After the clamp was closed, subsequent readings of bubble length and air temperature were made, and exudation pressure relative to ambient was calculated according to Gay-Lussac's law (Weast & Astle, 1981). Clamping itself did not affect bubble length. Care was taken when attaching the manometer so that a continuous column of liquid was present from stipe to bubble, and the attachment was gently secured with twist-ties to help guard against leakage. The relationship between root pressure and hydathode secretion was studied in three plants growing along a shaded stream in the cloud forest on a clear day. The entire above-ground portion of each individual plant was enclosed in a clear plastic bag and root pressure was monitored with a manometer attached to a single cut frond for each plant (Fig. 4A). Observations of xylem pressure and hydathode secretion were then made. Anatomical differences between the secreting and non-secreting hydathodes on these three plants and others were studied from hand sections and clearings from both fresh and material fixed in FAA. Clearings were made by treating the leaf with alcoholic sodium hydroxide followed by lactic acid. Observations were made on preparations left unstained or stained in toluidine blue. The distribution of lignin was determined from hand sections stained in phloroglucinol and concentrated HC1. Suberized and cutinized cell walls were identified using the I-KI/H2S04 test. S~~~~~~~~~~~~~~~~~~~~~~~~~... 68 AMERICANFERNJOURNAL: VOLUME 73 (1983) m:. .t xI _~ . .I t C~ll? - AL~ 9 ),Ai 'h:~..~ ks~' I~~~C~" _ "" ~,..,, ?., ,. :';' ?11 j 1 FIGS. 5-13. Developmentof Casparianstrip. All preparationswere treated in I-KI/H2SO4.FIGS. 5-8, 10, and 12 are from clearedpreparations; FIGS. 9, 11, and 13 are transversesections made parallel to the leaf margin. Arrows = Casparianstrip. FIGS. 5-9. Non-secretinghydathodes on maturefrond. FIG. 5. Vein subtendinga hydathodeand surroundedby endodermiswith fully developedCasparian strip. FIG. 6. Hydathodeviewed from the adaxial epidermisand focussed on the Casparianstrip borderingthe edge of the vein ending. FIG. 7. Highermagnification of the Casparianstrip as viewedin J. S. SPERRY:OBSERVATIONS ONBLECHNUM HYDATHODES 69 OBSERVATIONS Secretion.-Secretion was observedon developing,unenclosed fronds in foggy and rainyweather in the cloud forest and when individualcrowns were enclosed in plastic bags. In bagged plants, secretionwas initiatedon successive pinnae in an acropetaldirection so that the last hydathodesto begin secreting on developing fronds were those on the most distal