On the life cycle and germination of Hottonia Palustris L. in a wetland forest

Aquatic Botany, 35 (1989) 153-166

153

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

ON THE LIFE CYCLE A N D G E R M I N A T I O N OF H O T T O N I A P A L U S T R I S L. IN A W E T L A N D F O R E S T

THEO C.M. BROCK*, HANNEKE MIELO and GERARD OOSTERMEIJER

Hugo de Vries-Laboratory, University o/Amsterdam, Kruislaan 318, 1098 SM, Amsterdam (The Netherlands) (Accepted for publication 27 February 1989)

ABSTRACT Brock, T.C.M., Mielo, H. and Oostermeijer, G., 1989. On the life cycle and germination of Hottonia palustris L. in a wetland forest. Aquat. Bot., 35: 153-166. In 1986 and 1987, years that differed considerably in climatological conditions, the seasonal variation in biomass of Hottonia palustris L. was studied in the understory of a wetland forest. Furthermore, observations on its regeneration were made in the field and in the laboratory. The Hottonia population studied is characterized by early flowering, a large seed production, early development of maximum biomass, a well-developed vegetative regeneration and the ability to tolerate emergence and frost. The absence of overlying water in the summer of 1986 coincided with a relatively high biomass of Hottonia, while in the summer of 1987, when high water levels were recorded, its biomass considerably decreased. The seeds are characterized by their lack of innate dormancy, by the ability to remain viable after desiccation and by the ability to germinate over a relatively wide range of temperatures in the light and in an aerobic environment. No germination was observed under dark anaerobic conditions, and germinationpercentages were higher on a moist substrate than when the seeds were submerged. In the presence of overlyingwater most of the young seedlings rise to the water surface and may float there for several weeks. For their further development, however, a more or less permanent contact with the substrate seems to be required. Because of these life cycle characteristics, Hottonia palustris is well adapted to grow in the understory of wetland forests and in shallow waters that regularly dry up.

INTRODUCTION

Hottonia palustris L. (Fig. 1 ) is an aquatic vascular plant with a myriophyllid growth form (den Hartog and van der Velde, 1988), which has Europe and north Asia as its geographical range (Hegi, 1927). The species can be found in several types of shallow, circumneutral, low to moderately alkaline, meso- to eutrophic, stagnant to slow-flowing freshwater systems (see, e.g. Haslam, 1978; *Present address, Department of Nature Conservation, Agricultural University, Ritzema Bosweg 32a, 6703 AZ, Wageningen, The Netherlands.

0304-3770/89/$03.50

© 1989 Elsevier Science Publishers B.V.

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b~ C~

lcm

lcm

i

Fig. 1. General appearance of Hottonia palustris L. (a) Habit; (b) dissected flower; (c) mature fruit.

155 de Lyon and Roelofs, 1986). It tolerates emergence very well and under these circumstances develops a land form (Prankerd, 1911; Arber, 1920). In The Netherlands Hottonia palustris often occurs in localities characterized by upwelling water and a peaty substrate (Westhoff and den Held, 1969; de Lange, 1972), which may indicate that this aquatic macrophyte predominantly uses CO2 as its source of inorganic carbon. Besides its occurrence in more open freshwater ecosystems, this species can be found in shaded waters (Westhoff and den Held, 1969; Pott, 1980) and in the understory vegetation of wetland forests (Brock et al., 1989). Although the habitats in which Hottonia palustris occurs have been fairly well described in the literature, information on the survival biology of this plant is remarkably scarce. Knowledge of the survival biology of plants is important when explaining their distribution and their coexistence in communities (see, e.g. Grime, 1979; Verhoeven et al., 1982). Therefore, special attention was paid to the life cycle of Hottonia palustris when the authors were studying a wetland forest system in the eastern part of The Netherlands. The growth rhythm of Hottonia palustris in the wetland forest was studied by recording its seasonal variation in biomass and morphological differentiation in 1986 and 1987, years characterized by relatively dry and relatively wet climatological conditions, respectively. These aspects concern mainly the established phase of the plant. To gain a more complete insight into the life cycle of Hottoniapalustris, observations on its seed production, on seed germination, and on the development of the seedlings were made as well. SITE DESCRIPTION The field observations were carried out in a patch of wetland forest in the nature reserve "Het Molenven", municipality of Weerselo, The Netherlands (52 ° 19' N, 6 ° 47' E ). The patch of wetland forest selected had a surface area of 400 m 2. In August 1986, the tree layer of this patch was composed of Salix cinerea L. (coverage 45% ), A lnus glutinosa (L.) Vill. (20%) and Betula pubescens Ehrh. (1%). On average the tree canopy absorbed ca. 52% of incident light in winter and ca. 75% in summer (as measured around noon). In the understory vegetation several hydrophytes occurred, viz. Hottonia palustris (coverage 65% ), Lemna minor L. (5%), Ricciafluitans L. (5%), and CaUitriche platycarpa Kiitz. (3%). Emergent macrophytes on the forest floor covered ca. 5 % with species such as Lysimachia vulgaris L., Iris pseudacorus L., Carex elata All., Carexpseudocyperus L. and Carex curta Good. For a more detailed description of the vegetational structure of this community the reader is referred to Brock et al. (1989). The substrate in the patch consisted of a layer of peat, ca. 10 cm thick, resting on a deposit of sandy loam. The water table fluctuated considerably during the investigation period, ranging from ca. 25 cm above the sediment surface to

156 TABLE 1 Chemical properties (mean and range) of the overlying and dipwell water in a selected patch of wetland forest in the period April 1986-September 1987

Electric conductivity (ttS cm 2) pH Alkalinity (meql 1) P-PO,1 :~- (mg 1-~) N-NO:~- (mg 1-1) N-NO2- (mgl 1) N-NH4 + (mgl 1) Ca .-'+ (mgl -~) K + (mg1-1)

Overlying water (n=7)

Dipwell water (n=10)

121 (66-130) 5.9 (5.2-6.9) 0.18 (0.02-0.73) 0.10 (0.02-0.35) 0.09 (0.01-0.17) 0.01 (0.00-0.02) 1.4 (0.2-5.3) 10.0 (4.7-19.1) 2.2 (1.0-6.9)

347 (117-503) 7.0 (6.1-7.7) 1.59 (0.27-3.44) 0.09 (0.00-0.40) 2.01 (0.08-4.76) 0.11 (0.01-0.41) 0.5 (0.1-2.3) 56.9 (17.0-87.1) 2.0 (0.7-3.6)

far below ground level. The overlying water sampled in the patch can be characterized as slightly acid to circumneutral and relatively poor in electrolytes (Table 1). The alkalinity of the water was low and the concentrations of inorganic phosphorus and inorganic nitrogen moderate. The groundwater, sampled from a perforated PVC tube (with a total length of I m and inserted in the substrate to a depth of 0.5 m ), can be characterized as moderately buffered, circumneutral, and relatively rich in electrolytes. Nitrate and calcium levels were considerably higher in the dipwell water than in the overlying water. METHODS

Hottoniapalustris was harvested by means of a cylindrical core sampler (surface area 0.14 m2), which was pushed into the sediment to a depth of approximately 20 cm. In the wetland forest the roots of Hottonia did not penetrate deep into the sediment; they were predominantly confined to the top 10 cm. At each sampling date Hottonia palustris was harvested from eight cores. The plants from each core were placed separately in plastic bags and transported to the laboratory. Plants from each sample were freed of sediment and detritus by rinsing with tap water, packed in aluminium foil, dried at 105 °C for 24 h and weighed. Furthermore, at each sampling date 10 more or less complete plants of Hottonia palustris were harvested and divided into roots, stems, leaves and reproductive structures. The separated plant parts were then dried and weighed as described above. On 11 June 1987, when flowering was at its maximum, the density of inflorescences of Hottonia palustris was estimated by means of a 0.25-m 2 frame, which was thrown at random in the Hottonia vegetation on the forest floor. In each sample the number of inflorescences was counted and expressed as num-

157

bers m - 2. On the same day the mean number of seeds produced m - 2 was roughly estimated by counting the mean number of flowers and/or fruits per inflorescence, as well as the mean number of seeds per fruit. Hottonia seeds used for the germination experiments were harvested on 11 June 1987. A small sample of the seeds was used immediately in a germination experiment without pre-treatment, while the other seeds were stored under dry conditions in the dark at room temperature. In the various experiments seeds were used after different periods of dry storage. In one experiment seeds were pre-treated by storing them first under dry conditions at room temperature and then for a period of 14 days at - 18°C in a freezer. Germination tests were conducted in three climate chambers in which Philips TL fluorescent tubes provided a light intensity of ca. 75~E m -2 s -1. The climate chambers differed in their photoperiods and temperatures, in order to mimic more or less natural conditions: Chamber 1: 8-h photoperiod at 10°C, 16-h dark period at 5°C; Chamber 2: 12-h photoperiod at 15°C, 12-h dark period at 10 ° C; Chamber 3: 16-h photoperiod at 20 ° C, 8-h dark period at 15 ° C. In most germination experiments the seeds (50 per replicate) were placed in Petri dishes (8.5-cm diameter) containing 40 ml of tap water. Hottonia seeds introduced into the water do not sink immediately, and when no water movements occur they remain floating at the water surface until germination takes place. However, gently stirring the water or vibrations (e.g. caused by the cooling aggregate of the climate chambers) cause most of the seeds to sink within a week. Therefore, at the start of the experiments the seeds were pushed below the water surface so that in each test equal conditions for all seeds were obtained. In one experiment the Petri dishes contained moist filter paper instead of water, in order to mimic moist soil without overlying water. In some experiments the seeds were excluded from light by wrapping the Petri dishes in aluminium foil. In another test the seeds were placed in dark Winkler bottles containing water that was depleted of oxygen by means of N2 gas, in order to obtain a dark anaerobic environment. Germination of the seeds was checked weekly and all experiments lasted 7 weeks. Germination was registered when the emerging hypocotyl was visible by the naked eye. The final germination percentages were compared statistically by applying the one-way analysis of variance and Scheff~'s simultaneous test (Scheff~, 1959). The development of the seedlings of Hottonia palustris into full-grown plants was studied in an aquarium containing a 5-cm layer of peat and 5 cm of overlying water (collected in the wetland forest). This aquarium was placed in Climate Chamber 3, in which summer conditions were simulated. RESULTS

The annual variation in climatological conditions differed considerably between 1986 and 1987. In 1986 less than average rainfall fell in May-October,

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while temperatures were relatively high in May and June. The dry climatological conditions in 1986 resulted in a very low water table during the growth season; from mid-June until the beginning of December no overlying water was observed in the selected patch of wetland forest. In 1987, on the other hand, the period May-October was exceptionally wet, and overlying water was always present (Fig. 2). The seasonal variation in the biomass of Hottonia palustris is presented in Fig. 3. In 1986 the peak biomass of Hottonia was found at the end of May ( 173 _+36 g dry weight m-2 ); in 1987 this was somewhat later, in the beginning of June ( 197 _+32 g dry weight m-2 ). The annual variation in biomass differed considerably between 1986 and 1987. A relatively high biomass (ca. 130-140 g m -2) was recorded in June-December of 1986, when there was no overlying water, while in the period June-September of 1987, which was characterized by a high water level, the biomass decreased towards a minimum value of 61 + 25 g m -2. In the period January-March 1987 we did not harvest Hottonia palustris, because the water surrounding the plants was then completely frozen. The *SO cm

cm - SC

A'M'J "J "A'SON'DIJ "F'M'A'M'J J "AS

Fig. 2. Seasonal variation in the water table in a selected patch of wetland forest in the period April 1986-September 1987. gom -2

200

100 ~

~

-

~

AM-J J A'S'O'N'DIJ "F'M'A'ffJ J "AS Fig. 3. Seasonalvariation in dry weight biomass (mean_+SD, n -- 8) of Hottonia palustris in the understory of a selectedpatch of wetland forest in the period April 1986-September1987.

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plants embedded in the ice, however, were bright green and were apparently not damaged by the frost, since a relatively high biomass was recorded in April 1987, just after the frost period. In the summer of 1986 established plants developed a land form, characterized by smaller, stiffer leaves. Furthermore, we observed that Hottonia palustris expanded its area on the forest floor in this period; both detached shoots and seedlings settled successfully on several spots of emerged, but moist, bare peat soil. In 1987 we only found a few seedlings, but free-floating vegetative shoots were regularly observed. The seasonal variation in the relative biomass contribution of the different plant parts of Hottonia palustris is presented in Fig. 4. Floral parts were found in the period April-July. Reproductive structures were at their maximum at the same time as the peak biomass of the total plant was found. In 1986 the floral parts contributed up to 4% of the total biomass and in 1987 up to 6%. Leaves persisted throughout the year. The relative contribution of the leaves to the total biomass ranged between 36 and 51%, that of the stems between 26 and 43%, and that of the roots between 9 and 26%. It was particularly in the period July-November 1986, when the water level was below the sediment surface, that the biomass contribution of the roots was relatively high. The inflorescences of Hottonia palustris showed a patchy distribution on the forest floor, with densities ranging from zero up to 365 inflorescences m -2, and with a mean density of 56 m -2 (Table 2). On average, 16.6 flowers and/or fruits per inflorescence were counted and 68.6 seeds per fruit. In the wetland forest, fruit-ripening in Hottonia occurred entirely in the air and there were no indications that a large number of flowers did not set seed in 1987. These data suggest that the potential seed production of Hottonia palustris m -2 of the system is high and may exceed a number of 60 000. In the laboratory, seeds of Hottonia palustris germinated when incubated under relatively cold, short-day conditions and under relatively warm, long% 100

75

50

25

M J J "A "S "O'N'DIJ F ' M ' A ' M J "J A "

Fig. 4. Seasonal variation in the relative biomass contribution (dry weight) of different plant parts of Hottonia palustris in the period April 1986-September 1987.

160 TABLE 2 Density of inflorescences, number of flowers/fruits per inflorescence, and number of seeds per fruit of Hottonia palustris as measured on 11 June 1987 in a selected patch of wetland forest

Number of inflorescences m 2 Number of flowers and fruits per inflorescence Number of seeds per fruit

Range

Mean ± SD

n

0-356 1-36 49-93

56.0 ± 85.6 16.6 ± 8.5 68.6 ± 10.6

40 35 13

TABLE3 Time course of germination percentage (mean±SD, n=3)of2-month-oldseedsofHottoniapalustris in climate chambers differing in day-night regime and temperature. The seeds were placed in Petri dishes (diameter 8.5 cm) containing 40 ml of tap water Incubation time in days

Chamber I 8 h light at 10°C 16 h dark at 5°C

Chamber 2 12 h light at 15°C 12 h dark at 10°C

Chamber 3 16 h light at 20°C 8 h dark at 15°C

7 14 21 28 35 42 49

0 0 15±7.4 40±7.1 51±4.1 58±5.9 61±4.1

1±0.9 67±9.4 70±8.6 72±9.3 72±9.3 72±9.3 72±9.3

58±4.5 69±2.3 71±3.4 71±3.4 71±3.4 71±3.4 71±3.4

day conditions (Table 3). The germination rate, however, was highest in Climate Chamber 3, in which relatively warm summer conditions were simulated. The maximum germination percentage was attained here within 3 weeks of incubation. In Climate Chamber 2, with an intermediate day-night and temperature regime, the maximum germination percentage was attained within 4 weeks. The final germination percentages, however, did not differ significantly for Chambers 2 and 3 (P < 0.01 ). Climate Chamber 1, in which relatively cold short-day conditions were simulated, yielded the lowest germination rate. It is highly likely that the final germination percentage would have been higher here if the incubation period had been prolonged; nevertheless, no less than 61% of the seeds had germinated after 7 weeks. In all germination experiments conducted in Climate Chamber 3, the time course of germination was more or less the same as described in Table 3. Therefore, it suffices to present the final germination percentages when comparing the various experiments performed in Climate Chamber 3 (see Table 4). Here the seeds of Hottonia palustris showed a high germination percentage, regardless of whether they were incubated without pre-treatment (Experiment A), after pre-treatment by storage under dry conditions for as long as 15 months

161 TABLE4 G e r m i n a t i o n (%) of seeds of Hottonia palustris after 50 days of incubation in a climate c h a m b e r with a daily regime of 16 h light at 20°C a n d 8 h darkness at 15°C. T h e seeds incubated (50 per replicate) varied in t h e i r age a n d p r e - t r e a t m e n t . In all experiments, except G a n d I, Petri dishes (diameter 8.5 cm) containing 40 ml of tap water were used. In Experiment G the Petri-dishes contained moist filter paper instead of water a n d in E x p e r i m e n t I the seeds were placed in Winkler bottles Experiment

Period of dry storage (days)

Other pre-treatment

Special treatment

Germination % (mean + S D )

A B C D E F G H I J K

0 0 30 61 61 372 372 372 372 372 458

14daysat-18°C Experiment I -

Dark

89 20 91___2.9 71___3.4 72 76_+3.7 90 _+3.3 17_+5.9 0 80+6.7 74

Moist filter paper Dark Dark, anaerobic

(n=l) (n = 1 ) (n=3) (n=3) (n=l) (n=4) (n = 4 ) (n=4) (n = 4 ) (n=4) (n = 1 )

(Experiment K), or after pre-treatment by storage in a freezer for 14 days (Experiment E). The vitality of the seeds decreased slightly when they were stored under dry conditions for a relatively long time, since 30 days of dry storage (Experiment C) resulted in a significantly higher (P<0.01) germination percentage than a dry storage period of 61 (Experiment D) or 372 days (Experiment F ). The germination percentages for Experiments D and F, however, are not significantly different. The final germination percentage was significantly higher ( P < 0.01 ) when the seeds were incubated on moist filter paper (Experiment G) than after incubation under water (Experiment F). All experiments described above (which were performed in Climate Chamber 3) resulted in mean germination percentages of over 70%, and the final germination percentage was always attained within 3 weeks. Germination percentages were considerably lower (17-20%) when the seeds were incubated in the dark in aerobic water (Experiments B and H), and no germination was observed when the seeds were incubated in anaerobic water under dark conditions (Experiment I). However, when the seeds of Experiment I were incubated afterwards in the light and in aerobic water, a high germination percentage was recorded (Experiment J). The germination percentages for Experiments F and J are not statistically different. Several developmental stages of Hottoniapalustris, as observed in the aquarium in Climate Chamber 3, are presented in Fig. 5. At the start of the experiment all Hottonia seeds that were introduced into the aquarium floated at the

162

I¸ ii~,

c

/

/

~

\

d

bar is 1 m ~

Fig. 5. Several developmental stages of the seedlings of Hottonia palustris as cultured in a climate chamber (see text ). (a) Seed with emerging primary root (just germinated); (b) seed with emerging hypocotyl (age 2 days); (c) free-floating seedling with ring of root hairs at the root collar (age 7 days); (d) seedling with primary leaves and adventitious roots (age 40 days); (e) young plant of Hottonia palustris (age 2 months).

water surface, and it was observed that an air film surrounded the seed coat at this stage. It is likely that the net-like structure present on the seed coat (Fig. 5a) facilitates the existence of this air-film. The majority of the seeds remained floating. Within the first week the emerging hypocotyl became visible in most of the seeds that had sunk and those that floated (Fig. 5a,b). Within 12 days empty seed coats and free-floating seedlings (Fig. 5c) were observed. In this phase nearly all seedlings floated near or at the water surface. As long as overlying water was present in the aquarium the floating seedlings hardly developed any new leaves or roots. The development of a more elaborate root and shoot system (Fig. 5d) started when most of the overlying water in the aquarium had evaporated and the seedlings were in contact with the peat soil. In the aquarium this stage was reached after ca. 6-7 weeks. A month later plants with compound leaves (Fig. 5e ), resembling those of the adult Hottonia plants, were found. DISCUSSION

The population of Hottonia palustris in the selected patch of wetland forest was shown to have a perennial life cycle. A relatively large amount of biomass

163

was found during the dry summer of 1986 and during the winter of 1987. The plants survived emergence and were not damaged when they became embedded in the ice for several weeks. Maintaining a relatively high biomass in winter is certainly an advantage in species such as Hottonia palustris, which are characterized by early flowering and early development of maximum biomass. These characteristics may, at least in part, explain its ability to grow successfully in the understory vegetation of wetland forests, since the development of the biomass in Hottonia palustris starts in early spring when the forest floor is not yet heavily shaded by the tree canopy. Furthermore, water levels and dissolved nutrient concentrations are relatively high in this period. Although no data on seasonal changes in biomass of Hottonia palustris were found in the literature it is reported that also in other habitats, e.g. ditches and slow-flowing streams, this species also reaches its maximum coverage and size in late spring or early summer (Hoogers and van der Weij, 1970; Haslam, 1978). In the temperate regions of Europe relatively few aquatic vascular plant species (e.g. Ranunculus spp., Callitriche spp., Hottonia palustris) are characterized by a fast and early growth, and, according to Sculthorpe (1967), it is quite possible that these relatively few spring-flowering species are short-day plants. The present study demonstrates that the established plants of Hottonia palustris tolerate emergence very well, and that in the wetland forest the absence of overlying water during summer is apparently more favourable for sustaining a relatively high biomass than the presence of overlying water. When the plants emerge on the forest floor they develop a land form, characterized by smaller, stiffer leaves, and they invest more in their root biomass. The ability of several perennial aquatic vascular plants to survive emergence in their established phase, and their ability to develop land forms, is well documented (see, e.g. Arber, 1920; Sculthorpe, 1967; Brock et al., 1987). However, the phenomenon that during the growth season an aquatic macrophyte has a higher biomass when emerged than when growing in water was not reported in the literature we consulted. Environmental conditions in the understory of a wetland forest differ in several respects from those in more open shallow freshwater systems, e.g. in that the plants on the forest floor are shaded by trees and that the trees temper the diurnal fluctuations in climatological conditions. In the wetland forest studied the tree canopy on average absorbed ca. 75% of incident light in summer. When overlying water is present in the wetland forest the shade level for Hottonia certainly is higher, because light is also reflected and absorbed by the water column. Thus, the considerable decline in biomass of Hottonia palustris in the period June-September 1987 might be attributed, at least in part, to the high shade level caused by both the tree canopy and the water column. On the other hand, when overlying water is absent, some of the environmental features in the wetland forest can be considered more or less favourable for Hottonia palustris, such as the relatively high humidity of the air and the less

164

extreme fluctuations in air temperature on warm and sunny days. These conditions slow down the desiccation of Hottonia palustris during periods of drought. Furthermore, the absence of overlying water results in a higher level of light on the forest floor. The consolidation of a high biomass during the summer in emerged stands of Hottonia palustris will most probably not occur in more open freshwaters. According to den Hartog and van der Velde (1988), communities with Hottonia in these systems start their development in autumn, are wintergreen and decrease in biomass at the beginning of the summer; this decline is usually brought about by complete desiccation of the habitat. The mechanisms whereby plants regenerate account for many of the differences in ecology between species and between different populations of the same species (see e.g. van Wijk, 1988). Within this context it is important to make a distinction between vegetative and generative regeneration (Grime, 1979). In the wetland forest we observed that Hottonia palustris is able to produce new individuals by proliferation and subsequent fragmentation. Detached shoots were regularly observed floating in the water, and during the dry summer of 1986 we noticed that these shoots developed a more or less elaborate root system when in contact with the emerged peat soil. According to Hegi (1927) relatively many free-floating shoots of Hottonia palustris can be observed just after the flowering period; then secondary shoots become separated from the senescing main stem which supported the inflorescence. In the wetland forest vegetative expansion and successful establishment of detached shoots is of particular importance for the local maintenance and short distance dispersal of Hottonia palustris, while generative regeneration seems to be less essential for the local consolidation of the perennial population. Nevertheless, the seeds are certainly important for maintaining genetic diversity within the population, and as long distance dispersal agents. The present study demonstrates that in Hottoniapalustris light, aerobic conditions and moisture supply are the overriding determinants of the timing of germination. The germination percentages were low, though not negligible, in the dark, and high in the light, while no germination was observed under dark anaerobic conditions. Seeds of Hottonia germinated when submerged and they showed an even better germination when in contact with the air on a moist substrate. Furthermore, they were characterized by a lack of innate dormancy, the ability to germinate after being dried for at least 15 months, and the ability to germinate over a relatively wide range of temperatures. In addition, the seeds were not damaged by a short frost period or by remaining in a dark and anaerobic environment for at least 7 weeks. Because of these germination characteristics, Hottonia palustris is well adapted to inhabit shallow waters that regularly dry up. Furthermore, it can be argued that in an early flowering species such as Hottonia palustris, the chance that seedlings successfully establish is relatively large when germination is initiated at the beginning of the growth season. In this context the absence of innate dormancy can be considered an

165

advantage. However, when the majority of viable seeds germinate immediately after they are shed, a persistent reservoir of buried seeds will not develop, and a sudden change in environmental conditions just after mass germination might cause the death of all seedlings. This, however, may not be disastrous for a perennial population of Hottonia palustris with a well-developed ability to regenerate vegetatively. Hottonia is a plant that may appear suddenly along the margins of isolated freshwater systems, indicating its ability to become dispersed over relatively long distances (van der Pijl, 1972, and literature cited therein). The seeds of Hottonia palustris are small and relatively unspecialized, which, according to van der Pijl (1972), makes the dispersal by waterfowl quite possible. Hegi (1927) reports that in several areas of Europe the distribution of Hottonia palustris coincides with the main travel routes of waterfowl. These data suggest that epizoochory by birds is an important long-distance dispersal mechanism. A quantitatively more important method of dispersal over relatively short distances, however, may be transport via water, since both the detached ripe fruits and the seeds have the ability to float at the water surface for at least several days (Arber, 1920; Hegi, 1927; this study). Furthermore, when the seeds of Hottonia palustris germinate under water the young seedlings rise to the surface and may float there for several weeks (Hegi, 1927; Sculthorpe, 1967; this study). This adaptation might be important as a dispersal mechanism. According to Brockschmidt's (in Hegi, 1927) and our observations the seedlings hardly develop any new leaves and roots when floating at the water surface. A more or less permanent contact with the substrate seems to be required for its further development, indicating once more that very shallow sites or even emerged substrates are essential for a successful establishment of Hottonia

palustris. ACKNOWLEDGEMENTS

We thank the "Stichting Natura Docet" for the permission to perform this study in the nature reserve, Ben Oudejans and Dr. P. van der Molen for technical assistance, and Prof. Dr. C. den Hartog, Prof. Dr. T. van der Hammen, Dr. B. van Geel, Dr. F. Bouman and Dr. J. Klerkx for critically reviewing a previous draft of this paper. REFERENCES Arber, A., 1920. Waterplants, a Study of Aquatic Angiosperms. Cambridge University Press, Cambridge, 436 pp. Brock, T.C.M., van der Velde, G. and van der Steeg, H.M., 1987. The effects of extreme water level fluctuations on the wetland vegetation of a nymphaeid-dominated oxbow lake in The Netherlands. Arch. Hydrobiol. Beih., 27: 57-73. Brock, T.C.M., van der Hammen, T., Van der Molen, P.C., Ran, E.T.H., Van Reenen, G.B.A. and

166 Wijmstra, T.A., 1989. An account of the history, flora and vegetation of nature reserve 'Het Molenven', Province of Overijssel, The Netherlands. Proc. Kon. Ned. Akad. Wet., B, 92: 1-46. De Lange, L., 1972. An ecological study of ditch vegetation in the Netherlands. Thesis, University of Amsterdam, 112 pp. De Lyon, M.J.H. and Roelofs, J.G.M., 1986. Waterplanten in relatie tot waterkwaliteit en bodemgesteldheid, Deel I. Report of the Laboratory of Aquatic Ecology, Catholic University, Nijmegen, The Netherlands, 106 pp. Den Hartog, C. and van der Velde, G., 1988. Structural aspects of aquatic plant communities. In: J.J. Symoens (Editor), Vegetation of Inland Waters. Kluwer Academic Publishers, Dordrecht, pp. 113-153. Grime, J.P., 1979. Plant Strategies and Vegetation Processes. Wiley, Chichester, 222 pp. Haslam, S.M., 1978. River Plants. Cambridge University Press, Cambridge, 396 pp. Hegi, G., 1927. Illustrierte Flora von Mittel-Europa, Band V, 3. Teil, J.F. Lehmanns Verlag, Mtinchen, pp. 1569-2251. Hoogers, B.J. and Van der Weij, H.G., 1970. De ontwikkeling van slootvegetaties. De Levende Natuur, 73: 13-20. Pott, R., 1980. Die Wasser- und Sumpfvegetation eutropher Gew~isser in der Westf~ilischen Bucht. Abhandl. Landesmus. Naturk., 42, Mtinster, 156 pp. Prankerd, T.L., 1911. On the structure and biology of the genus Hottonia. Ann. Bot., 25: 253-267. Scheff~, H., 1959. The Analysis of Variance. A Wiley Publication in Mathematical Statistics, John Wiley, New York, 477 pp. Sculthorpe, C.D., 1967. The Biology of Aquatic Vascular Plants. Edward Arnold, London, 610 pp. Van der Pijl, L., 1972. Principles of Dispersal in Higher Plants. Springer-Verlag, Berlin, 153 pp. Van Wijk, R.J., 1988. Ecological studies on Potamogeton pectinatus L. I. General characteristics, biomass production and life cycles under field conditions. Aquat. Bot., 31: 211-258. Verhoeven, J.T.A., Jacobs, R.P.W.M. and van Vierssen, W., 1982. Life-strategies of aquatic plants: some critical notes and recommendations for further research. In: J.J. Symoens, S.S. Hooper and P. Compare (Editor), Studies on Aquatic Vascular Plants. R. Bot. Soc., Belgium, Brussels, pp. 158-164. Westhoff, V. and den Held, A.J., 1969. Plantengemeenschappen in Nederland. Thieme, Zutphen, 324 pp.