[机经背景分类] 植物类

MANGROVE
Mangroves produce from their trunks aerial roots that
become embedded in the mud and form a tangled network;
this serves both as a prop for the tree and as a means
of aerating the root system. Such roots also form a
base for the deposit of silt and other material
carried by the tides, and thus land is built up which
is gradually invaded by other vegetation. Some
mangrove species lack prop roots but have special
pores on their branching root system for obtaining
air. The mangrove fruit is a conical reddish-brown
berry. Mangroves have been harvested destructively on
a large scale; the bark is a rich source of tannins,
and the wood is used for wharf pilings and other
purposes.
红衫树
對於大部分的植物來說，種子是植物的生命之源，在其萌芽時需要充分的水分與氧氣，然而在河海交接的半鹹水沼澤中，紅樹林是如何適應並繁衍的呢？因在極度缺氧與鹽度高的沼澤軟泥上，既不適合種子發芽，也不利於幼苗的生長，因此紅樹科的植物就發展出這種先發芽、後落地生根的繁殖方式，以克服沼澤地的惡劣環境，也就是我們所謂的「胎生」。
紅樹林植物，也是經由開花、結果，來產生它們的下一代「種子」，但是即使種子成熟了，也不會從樹上脫落，相反的，包藏在果實體內部的胚芽開始發育，漸漸地變為帶有胚莖的「筆狀胎生苗」；胎生苗從母株吸收營養，並利用胚莖上的皮孔呼吸，繼續成長到成熟就可脫離母樹，直直落下並插入軟泥中，開始發根且長出新葉。水筆仔的胎生苗發育期大概約七、八個月左右會成熟，所以我們可以在夏末開花後，觀察到結果、胎生苗的發育而一直到第二年春天，胎生苗才會落下。  

LICHEN
usually slow-growing organism of simple structure,
composed of a fungus (see Fungi) and a photosynthetic
green alga (see algae) or cyanobacteria living
together in a symbiotic relationship and resulting in
a structure that resembles neither constituent.
Lichens commonly grow on rocks, trees, fence posts,
and similar objects. Lichens require no food source
other than light, air, and minerals. They depend
heavily on rainwater for their minerals and are
sensitive to rain-borne pollutants.
Before the discovery of aniline dyes, lichens were
much used for silk and wool dyes. Others have been
used in perfume manufacturing and brewing.
Lichen 青苔, 地衣, 苔藓
藻类和真菌共生的特殊的植物类型。组成地衣的藻类主要有念珠藻(Nostoc )﹑共球藻 (Trebouxia )和堇青藻(Trentepohlia )等蓝藻和绿藻。组成地衣的真菌主要是子囊菌﹐少数为担子菌和极个别的藻状菌。地衣中的藻类能通过光合作用制造有机物﹐供自身和菌类的需要﹐菌类能吸收水分和无机盐﹐供藻类进行光合作用并使地衣保持一定湿度。这种共生关系使地衣具有独特的形态﹑结构﹑生理和遗传的生物学特性。
地衣的形态﹐按生长型可分为三﹐即叶状﹑壳状和枝状﹐也有一些种类属于过渡类型。典型叶状地衣的构造分上皮层﹑藻胞层﹑髓层和下皮层﹐以假根或脐固着于基质。典型壳状地衣的构造无皮层或只有上皮层﹐以髓层的菌丝固着于基质。枝状地衣的构造呈辐射状﹐有外皮层﹑藻胞层和髓。
地衣对大气污染的反应十分敏感﹐1968年在荷兰瓦赫宁根举行的大气污染对动﹑植物影响的国际会议上﹐与苔藓植物一起被推荐为大气污染的指示植物(见大气污染指示生物)。使用方法一般是对地衣在污染区的种类﹑数量和分布进行调查或用人工移植进行定时定点监测。地衣对二氧化硫 (SO )的敏感性依壳状地衣﹑叶状地衣和枝状地衣的顺序递增。有人据此将无任何地衣存在的区域划为SO 严重污染区﹐只有壳状地衣的区域划为SO 重污染区﹐有叶状地衣而无枝状地衣的区域划为SO 轻污染区﹐有枝状地衣正常生长的区域划为无SO 污染的清洁区

Sunflower
common name for annual and perennial herbs of a genus of the family of composite flowers. The genus, which contains about 67 species, is thought to be native to South America originally, although sunflowers are now distributed almost worldwide. Some of the tall-growing forms may attain a height of 3 m (12 ft). The large, solitary blossom, sometimes as large as a meter in diameter, is composed of yellow ray flowers and a central disk of either yellow, brown, or purple flowers, depending on the species. The daily orientation of the flower to the sun is a direct result of differential growth of the stem. A plant-growth regulator, or auxin, accumulates on the shaded side of a plant when conditions of unequal light prevail. Because of this accumulation, the darker side grows faster than the sunlit side. Thus, the stem bends toward the sun.

Plant adaptation to the desert
Cactus adaptations.
The secret to the superior endurance of cacti lies in their adaptations. Over millions of years, through natural selection, only the strongest and best adapted species survived.

As you know, it is very dry in the desert. Plants that adapt to this are known as xerophytes (from xeros, dry and phyton, plant). There are plants that avoid the dry season by sprouting from seed just after the spring rain and growing very fast so that by the time the dry season comes, they have already produced a lot of seeds and died. These seeds lie on the soil for the dry season and sprout again in spring and the cycle repeats. Other xerophytes simply drop their leaves and stay dormant for the winter. But there is another special type of xerophyte which stores water in its fleshy tissues. Such plants are called succulents (from succus, juicy). The cactus is a typical example of a succulent.

If you cut a cactus open, you see a juicy, slimy tissue. This is where the moisture is stored for the dry season. The part between the middle circle (or pith) and just under the very green part of the plant (or palisade parenchyma) just under the skin is allocated for the storage of water and food for the plant. This is a type of spongy parenchyma and can take up up to 85% of the plant's volume. This is a major adaptation in the desert. Because the plant remains completely alive during the dry season and there is no need for it to dry up and lose everything, makes it possible for the plant to grow to large sizes. Another advantage is that the plant retains supplies (in the form of starch) for the winter so that it can flower right away in spring without accumulating more supplies (as most plants need to do in spring). The whole purpose of storing supplies for the winter is mostly to energize flowering in spring but it also lets the cactus start growing much sooner.

Flowering plants breathe and transpire (evaporate water from their surface) through closeable microscopic pores called stomates on the leaves or stems. To do this, their pores have to be open. In most plants these are open all day and on warm nights. But for cacti this is inconvenient as in daytime it is very hot and thus the plant would lose a lot of water through evaporation. So the cactus must close them in the daytime. But then it cannot breathe or photosynthesize (the process where sugars are made from carbon dioxide and water and releasing oxygen using the sun's energy). Succulents have an adaptation to that. Their stomates are closed during the day and are open at night, when it is not that hot and store carbon dioxide in its tissues as crassulean acid and then turn it back to carbon dioxide in the daytime. This process is called crassulean acid metabolism or CAM and it is a very smart way of respiring in the desert.

If we look at the outside of the plant, we notice that there is a tough leathery skin covering the plant, we can also notice the presence of ribs and spines and sometimes fur. These are all very smart adaptations. They serve mainly for surviving heat but are also used as defense.

The tough leathery skin is very impermeable to water, thus reducing evaporation from the surface of the plant. This skin often has a layer of plant wax on it which is often lightly coloured (Pilosocereus azures is an example of a plant with such wax), white or blue. This reflects light and also reduces evaporation from the inside.

The ribs are special structures that are also used for enduring extreme heat. The ribs (and spines) trap wind so that the plant is enveloped in a layer of extremely still air, and this is a very important factor in reducing evaporation. On a very windy days even the ribs don't help and cacti sometimes wilt because of high water loss.

The spines have different functions. They not only help shade the plant from the sun but are also known to help the cactus absorb water. They do it like this. On cool nights, dew settles on the spines of the plant. The spines are actually known to draw droplets of water towards the areole (the point out of which the spines grow) and here the droplets are absorbed. You can try this at home. Spray the plants with a very fine mist of water and watch what happens to the droplets that settle on the spines. They literally get attracted to the areole along the spine. The spine's structure allows them to do this. Even spines pointing downwards seem to suck the droplets up themselves.
   
Adaptation features are visible in this
Pilosocereus glauchochorous. Notice the
spines, ribs, fur and wax (the blue coloration).  The top of a typically adapted plant.  

Some plants have fur, sometimes all over the plant, sometimes only near the top. This fur shades the plant even further and is also known to attract water towards the areole. Some plants only have fur near the top. This is very beneficial because the top of the plant is very sensitive to sunlight, new tissues get formed there. Young areoles, with their spines not even wooded yet can get dried up completely in the sun. When an areole is born near the top of the plant, it starts developing spines. At this time the fur appears as well. This fur accompanies the areole as it moves down the plant, shading the growing point inside. By the time the areole is about 15cm away from the top, the fur wears out completely and the now inactive areole gets exposed to the sun.

As for the roots of cacti, they are also fully adapted to living in the desert. Some species (especially plants from very dry deserts) have very shallow root systems that spread very far from the plant. This way the plant can take advantage of tiny amounts of moisture from dew or light rain as the roots spread far away and are very shallow (less than 10cm deep while spreading up to 5 metres from the parent plant). On the other hand, some cacti send their roots deep down (like many Echinocacti) to reach the ground water.

Rainforest cacti often have aerial roots that can collect water all the time when it rains (and it rains very often in South American forests).

The shape of cacti itself is an adaptation. You may have noticed that cacti have a barrel like or candle like shape. This allows for maximum internal volume with a minimum surface area, which is also very smart adaption as a cactus can store a lot of water and have a small external surface area to reduce water loss.

Rainforest Layers
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High temperature and rainfall production, more or less uniform throughout the year, characterize the rainforest worldwide. The water that is required to maintain this environment also threatens its existence. If left uncontrolled, the water that constantly bombards the soil would leech out most of the valuable nutrients. Plants and animals in this environment have developed adaptations to deal with the constant deluge and to compete successfully for nutrients.
A closer look at the rainforest reveals that it is actually composed of four layers or communities. Each layer has a unique set of environmental conditions and organisms adapted to them. Read the descriptions below and watch for these layers and the infinite variety of life forms that occupy them as you visit La Selva.
The Emergent Layer
The tallest trees are the emergents, towering as much as 200 feet (60 m) above the forest floor with trunks that measure up to 16 feet (5 m) around. These huge trunks are usually supported by buttress roots to brace against the high winds. Most of these trees are broad-leaved, hardwood evergreens. They are exposed to greater fluctuations of temperature, wind and rainfall than are their smaller companions. To hold water, leaves often have thick, waxy layers. Emergents may take advantage of the greater air movement above the canopy by developing winged seeds or fruits that are dispersed by wind to other parts of the forest. Sunlight is plentiful and animals such as eagles, monkeys, butterflies, insect-eating bats and snakes inhabit this layer, some never venturing below it.
The Canopy
The primary layer of the rainforest, the canopy, extends beneath the emergents, rising to 150 feet (45m). Most canopy trees have smooth, oval leaves that come to a point. A possible explanation for this adaptation is that they shed rain quickly, discouraging the growth of lichens and mosses. In cloud forests such as La Selva, the canopy is lower and more dense, formed by smaller trees with twisted crowns of tiny, leathery leaves. At these higher elevations, the leaves have developed a highly reflective property that protects them from the higher levels of intense radiation. This almost solid green shield filters out 80% of the light, preventing its transmission to the forest below.
Photosynthesis is everywhere. Flowers and fruits abound. Many species flower simultaneously, aiding cross-pollination. In some species, flowers are produced on the trunks, making it easier for bat pollinators to find their way to the flowers. Monkeys, sloths, bats, treefrogs, ants, beetles, parrots, hummingbirds and snakes, to mention a few, can be found here, often never touching the ground during their lifetime. Epiphytes, some 28,000 species worldwide, use every tree surface as a place to live. Hollow trunks of trees and pools of water in bromeliads often are micro-communities within the Canopy.
The Understory
This area gets only 2-5% of the sunlight available to the canopy. This limited light encourages the plant residents to devise unique ways to survive, such as the solar-collecting dark green leaves. Plants that survive in the understory include dwarf palms and soft-stalked species of families, such as the ginger family, acanthus and prayer plant or Maranta. These plants seldom grow to more than 12 feet (3.5 m) in height. Understory plants have a more difficult time with pollination because of the lack of air movement. Most rely on insects. Some produce strong smelling flowers, others produce flowers and fruit on their trunks. This phenomenon, known as cauliflory, makes them more conspicuous to aid the process of pollination and seed dispersal. Many animals live here, including snakes, frogs, parakeets, leopards or jaguars and the largest concentration of insects.
The Forest Floor
Almost no plants grow in this region of 0-2% light and 100% humidity. The few flowering plants that live here tolerate deep shade. The floor itself is covered with a litter of rapidly decomposing vegetation and organisms that break down into usable nutrients. A leaf that might take one year to decompose in a temperate climate, will disappear in just six weeks on the rainforest floor. A high proportion of the nutrients in the system are locked in the large biomass (trees and other plant storage systems). There is heavy competition for these nutrients. This is why many trees are so shallow-rooted. Large mammals, such as tapirs, forage for roots and tubers. Insects, including termites, cockroaches, beetles, centipedes, millipedes, scorpions and earthworms, along with the fungi, use the organic litter as a source of food.

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