本研究探討具有兩棲特性的植物銅錢草(Hydrocotyle verticillata )，當其生長的水陸環境及水位高度發生變化時，其生理及形態的變化。田野調查發現生長於水深處(水位高度超過30 cm)的銅錢草其葉柄長度較水淺處(水位高度未超過30 cm)的銅錢草葉柄長度更長；生長於三種環境下(陸生、水生挺水和水生浮水)的葉片間其葉片上下表皮氣孔密度比值沒有顯著差異，且不論其葉上表皮氣孔密度或其葉下表皮氣孔密度，也都沒有顯著差異，顯示環境改變對於銅錢草葉片氣孔在上下表皮的分佈沒有影響。
實驗設計人工栽培陸生與水生銅錢草，在改變水陸環境前後測量淨光合作用率、氣孔導度、葉片細胞間隙二氧化碳濃度與葉綠素螢光參數Fv/Fm，並在提高水位的高度栽培前後測量其葉柄長度。研究結果發現，不論改變銅錢草栽培環境至水域或陸域環境下，其Fv / Fm皆無明顯變化，顯示環境中水分的增加或減少並未使銅錢草遭受很大的逆壓。當陸生個體移至水域三週後，平均淨光合作用率由0.98 ± 0.72 μmolCO2‧m-2s-1增高至3.71 ± 1.29 μmolCO2‧m-2s-1，增加量明顯高於維持在陸域環境下的陸生個體。這可能與氣孔導度和生化反應的增加有關。然而維持在陸域的個體其Fv / Fm由0.72 ± 0.05下降至0.61 ± 0.08，顯示其生化反應並無提升，故其淨光合作用率的增加可能是與氣孔導度增加有關。反之，當水生個體移至陸域三週後，其淨光合作用率的增加量和其他維持在水域的水生個體沒有明顯差異。根據其氣孔導度無明顯變化、細胞間二氧化碳明顯減少，以及其Fv/Fm無明顯變化，可推論其淨光合作用率的增加可能是與生化反應的增加有關，也可能與水生植株較廣大的根系發育有關。此外，銅錢草葉柄在環境由正常水位增加至高水位後延長57%，故推論銅錢草可藉由增加葉柄長度，以利水平舉高葉片，避免葉片浮水或完全沉水，使維持適當蒸散作用和氣體交換，以利進行光合作用。
根據以上結果，顯示水域環境更有利銅錢草行光合作用，且當環境水分減少時，水生銅錢草也不容易陷入缺水逆境。由本研究得知銅錢草對於時而陸生、時而挺水，甚至是淹水的環境變動有很強的適應力，故當棲地淹水或水位上升的情況下，銅錢草可能會有更廣泛地分佈。至於銅錢草更廣泛地分佈是否將威脅原生物種、乾旱是否能限制其族群分佈，都將值得進一步的關注。

This study examined how morphology and physiology of the amphibious plant, whorled marsh pennywort (Hydrocotyle verticillata Thunb.), respond to changes in aquatic and terrestrial environments and changes in water level. The ratio of stomatal density between upper and lower epidermis shows no significant difference among 3 groups: terrestrial leaf, emergent leaf, and floating leaf. There was also no significant difference in stomatal density of upper or lower epidermis among the 3 groups. The result indicates that these environmental changes do not affect the ratio of stomatal density between upper and lower epidermis of Hydrocotyle verticillata. Net photosynthesis rate(Pn), stomatal conductance(gs), intercellular CO2 concentration(Ci), the chlorophyll fluorescence (Fv / Fm ratio) and petiole length were investigated in cultured plants. After growing the terrestrial individuals in aquatic environment for 3, the Pn increases to 3.71 ± 1.29 μmolCO2‧m-2s-1 from 0.98 ± 0.72 μmolCO2‧m-2s-1. The increment of the terrestrial plants after aquatic treatment is significantly higher than the increment of plants in the control group, possibly due to the increase of the gs and biochemical reactions in the treated group. The Fv/Fm ratio of the terrestrial individuals decreases to 0.61 ± 0.08 from 0.72 ± 0.05, suggesting no increase in biochemical reactions. Thus, the increase in Pn may be related to the increase in the gs of the terrestrial ones.
On the contrary, the Pn and the Fv/Fm ratio show no significant difference between the aquatic plants and the aquatic ones after growing under terrestrial environment for 3 weeks. There is no significant change in the gs and the Fv / Fm ratio but there is a significant decrease in Ci following the treatment. Thus, the increase in Pn may be related to the increase in biochemical reactions of the terrestrial ones. Besides, the petiole length of the aquatic plants extends by 57% after 3 weeks treatment with raising water level. The petiole length extension shows significant difference between the high water level ones and the regular water level ones. Apparently the blade can be lifted up vertically to avoid floating or being submerged. It is inferred that Hydrocotyle verticillata maintains proper transpiration and gas exchange by means of adjusting increase of petiole length.
In summary, Hydrocotyle verticillata has higher capacity of photosynthesis in aquatic environment. Hydrocotyle verticillata can adapt changing environment of terrestrial, aquatic and even high water level condition with no sign of physiological stress. Hydrocotyle verticillata could grow extensively when facing water logged habitat or changes in water level. Whether or not the wide distribution of Hydrocotyle verticillata could threat the growth or survival of native plant species, and whether or not its distribution is limited by drought, deserve further investigations.

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