沿岸植物的適應 Coastal Plant Adaptations
另外一些紅樹，例如海漆（Excoecaria agallocha）（圖二）、木欖（Bruguiera gymnorrhiza） 則會把鹽分儲存在老葉裡，待樹葉枯萎脫落時把鹽帶走。
出水通氣根由植物泥下的纜狀根延伸而成，向上伸出泥土表面，可以達30厘米高，常見於白骨壤（Avicennia marina ）（圖三、四）。膝狀根與出水通氣根原理相似，只是形狀不同，木欖和秋茄（Kandelia obovata）都有膝根。
為了減少與強風接觸，沙灘上的植物大多匍匐而生，甚至只把葉的一部分露出地面，而地底根部卻非常深而廣，以抓緊泥沙，同時尋找水源。 匐枝栓果菊（Launaea sarmentosa）便是一個好例子（圖十）。
It is not difficult to find plants such as mangroves and sandy beach plants in coastal areas. However, to survive in this environment, there are some challenges the plants must face Firstly, a high soil and water salinity. Next, a low oxygen level because of high soil moisture. Then, the difficulty of obtaining fresh water from the soil. Fourthly, the difficulty in developing new plants from seeds. Last but not least, very strong winds. Surprisingly, coastal plants have displayed various measures to counter such a harsh environment.
1. Lowering salinity
Plants living along intertidal zones have to face high but ever-changing salinities. During high tides, the soil will be submerged in seawater, while during low tides, evaporation causes soil salinity to become even higher. To maintain a stable salinity in its own body, intertidal plants have two ways to choose: avoid absorbing too much salt, or expelling excess salt.
Some mangroves have special filtration mechanisms in their roots or have wax-covered roots to avoid most of the salt intake.
Many others, such as the River Mangrove (Aegiceras corniculatum), have salt glands under the leaves to expel excess salt. Often salt crystals can be seen on the leaf surfaces as white patches (Fig. 1).
Some other mangroves, like the Milky Mangrove (Excoecaria agallocha) (Fig. 2) and the Many-Petaled Mangrove (Bruguiera gymnorrhiza), store the salt in their older leaves. When those leaves wither and fall, the salt can be taken away with them.
2. Obtaining oxygen
In times of hide tides, gaps between soil particles will be filled with seawater, causing the soil to maintain a prolonged low oxygen level. Thus, plants cannot obtain enough oxygen just from soil.
To solve this problem, many mangroves have aerial roots to aid in absorbing air. The aerial roots are covered with lenticels for gaseous exchange. Moreover, inside the roots are spongy tissues with a lot of spaces for easier locking and transporting of oxygen. Aerial roots can be classified into different types, such as pneumatophores, ‘knee joint’ roots and propping roots.
Pneumatophores extend from the cable roots beneath the soil. They rise from the soil and can reach a height of 30 cm. They can be commonly seen in the Grey Mangrove (Avicennia marina) (Fig. 3 and 4). ‘Knee joint’ roots have a similar function despite the difference in shape. They are possessed by the Many-Petaled Mangrove and Kandelia obovata.
Propping roots grow from the stem downwards into the soil. Their main function is to support the plant but they help in breathing as well.
3. Keeping moisture
To coastal plants, freshwater is a precious resource. Therefore, they strive to reduce water loss. This is achieved by developing adaptation features on their leaves.
Many mangroves have leaves with a thick waxy cuticle and epidermis, in order to reduce the rate of transpiration of water from the leaf surface (Fig. 5).
Austral Seablite (Suaeda australis) have small but succulent leaves (Fig. 6). This allows storage of plenty of water and at the same time minimizes the area for water evaporation.
The leaves of some mangroves are lighter-coloured on the abaxial (lower) side than on the adaxial (upper) side (Fig. 7). A lighter colour allows better reflection of sunlight, which leads to a lower temperature and hence slower evaporation. Some leaves have a layer of thin hair to prevent water from escaping.
Other than the above mentioned, some plants have sunken stomata on their leaves which form pocket-like openings. This further reduces evaporation rate. Other measures include orienting the leaves away from sunlight.
4. Producing offspring
Owing to the instability of soil conditions in intertidal zones, plants have to think of ways to increase the survival rate of their seeds. One of these ways is vivipary. Even before the fruit detaches from the mother tree, the seed inside has started to germinate (Fig. 8). When mature enough, the seedling leaves the tree with the fruit (called a dropper). This way, once the seedling finds a suitable location it can quickly root itself to the ground and start growing without having to germinate.
Most mangrove droppers have high buoyancy, like those of Kandelia obovata. Its droppers can flow vertically in the water, so once it finds suitable soil, it can easily get caught in the mud and grow.
5. Withstanding strong winds
Winds are often stronger in coastal areas, especially beaches. Plants do not only have to avoid being blown uprooted, but they also have to avoid getting covered by the sand.
In attempt not to face strong winds directly, most plants on sandy beaches grow creeping near the ground. Some even only reveal the leaves above ground, while having a wide and deep root system buried beneath the sand. The root system ensures good anchorage on the sand and look for water at the same time. One good example of such plants is the Beach Launaea (Launaea sarmentosa) (Fig. 10).
Stems of Beach Morning Glory (Ipomoea pes-caprae) lie low on the ground and grow horizontally to stay away from strong winds. A lot of adventitious roots sprout from the stems as well to provide better ground gripping ability (Fig. 11).
Beach Silvertops (Glehnia littoralis) have low-lying leaves but slightly elevated flowers (Fig. 12). This is to prevent the flowers from being buried by the sand blown up by strong winds.
Despite the harsh environment, many plants in Hong Kong can still thrive and display a strong perseverance and liveliness. Besides, they also provide shelter for different wildlife. Therefore, apart from admiring these plants, we should learn to treasure them too.