摘要或動機

Marsilea quadrifolia, an aquatic perennial rhizomatous fern, is widespread on wetlands in Taiwan. A normal fern frond has the clusters of spore capsules on the back, yet Marsilea quadrifolia reproduces clusters at petiole base from sporocarps which are axillary, pedicellate, ovate and hairy; both microsporangia and megasporangia can be found in sporocarps. It is highly polymorphic. The sterile fronds are erect when growing in mud, and floating in water, and they produce leaflets in the shape of four-leaf clovers. Mature leaves are divided into water leaves and terrestrial leaves according to living environments. Water leaves are divided into submerged leaves, floating leaves, and emergent leaves. These four kinds of leaves have obvious differences in stoma density. The submerged leaves have the least starch which means they have the minimum photosynthetic efficiency. The flexible leafstalks allow the plants to adapt to small changes in water level, while keeping their leaves on the water surface to access light and carbon dioxide. Thus, heterophylly may be seen as an adaptive mechanism which is sensitive to some environmental parameters and that responds accordingly to maximize the capability for survival. The terrestrial leaves have periodic sleep movement according to the alternate changes of day and night. The biggest opening angle is related to a definite leaf closing time cycle. Light interference during the dark period, continuous lighting, or the reversing of day and night all interfere the opening time and speed of them.南國田字草是一種浮葉性、多年生水生蕨類，匍匐莖細長橫臥在水中泥土或濕地裡，葉四枚對生於葉柄頂端，依生長環境不同，可分為水生葉及陸生葉，水生葉又分沉水葉、浮水葉及挺水葉三種。孢子囊群聚集特化成堅硬的孢子囊果，長於葉柄基部，孢子有大、小孢子兩型。四種異型葉在內部構造、氣孔分布密度上皆有相當的差異，以適應環境的變化，葉片澱粉含量以沉水葉最少。南國田字草陸生葉隨著晝夜交替，具有週期性的睡眠運動，每天葉片開啟的最大角度和閉合時間有一定的時間範圍。黑暗期受光照干擾或連續照光、日夜顛倒，皆影響葉片的開閉時間與速率。