Super Plants of the Waterless Habitats: The Cactaceae
Environment. Epiphytes. Adaptation. Morphology. Photosynthesis. Transpiration. ABA (Abscisic Acid). CAM (Crassulacean Acid Metabolism)
Survival in the xerophytic environment becomes the norm rather than the rule, plants has to adapt a strategy in order to survive, that their habitats control their lives. Among the most hostile habitat is the Desert. Daily temperature variation in the desert is about 22°c, moreover, it can rise up to 55°c at midday enough to wilt all vegetation.The amount of rainfall received is very low. Precipitation is about 250 mm annually, but there are areas that even received as low as 40 mm per year.The amount of moisture received is not even enough to appease the already thirsty plants and shower the water- less soil. On this account, only plants that possessed the special type of adaptation can survive in this hostile land. Despite their waterless habitat, Cactus employs a variety of strategies in order to survive in a hot waterless environment of the desert. To perpetuate their species, Cactus shows bright and colorful flowers and nectaries that attract nocturnal and diurnal pollinators How this unique plants survived? this article discusses some of their strategies.
Distribution and Habitat
The flowering plants or Angiosperms are composed of vast collections of flowering plants that inhabit the earth.Among this plants are the Cactus or Cacti (plural). considered unique in Angiosperms because of the nature of their habitat. Worldwide , Cactus is cosmopolitan to the Americas (North and South) and some parts of Africa (see map). Although they prefer the harsh environment of the desert ,there are species that also thrives well in tropical conditions where moisture is abundant . Some Cactus are Epiphytes (lacking true stem) rises from the ground by attaching themselves to other plants .
The origin of Cactus can be traced in Americas, where a large number of their species are found.Some cacti are even found below sea level as in the case of Death Valley in California to the driest place on Earth, the Atacama desert, to the high elevation mountain of the Andes at 15000 feet above sea level .Today Cactus are no longer confined to their origin but are already found in Parks, Botanic Gardens, Arboretums, and Private Collections, and commercially sold in Pots or in Jars in florist shop worldwide.
Deserts are among the hostile landforms on earth. Under hot and waterless conditions of the desert , a plant must employ a variety of tactics in order to survive .For instance, Xerophytes (plants that grow in arid or dry habitat, including Cactus) under high temperature employs leaf rolling or leaf folding or appeared wilted by dropping its leaves. In this way, plants avoid exposing large parts of its body to high temperature or radiation from the sun, otherwise, thermal damage to photosynthetic tissue may occur that tend to threaten the survival of the plant .Chemical signals like Abscisic acid(.ABA) is synthesized in the mesophyll ( photosynthesizing tissue ) during water stress. The stomates responded by closing the stoma (aperture) through the guard cells. The closing of stomata (singular) further prevent water loss due to transpiration, thus conserving precious water (see physiology).How Abscisic Acid (ABA) is synthesized in the leaves are now fully understood , but roots under extreme water stress also send chemical signals to the leaves to close their stomates, a kind of long-distance signalling in the roots in response to receding water supply in the soil.Other xeric species of the desert employs temporal separation of carbon dioxide (CO2) acquisition and fixation (eg. Cactus). Normal plants acquire their CO2 during daytime only when the stomates are open and stopped CO2 absorption when the stomates are closed during night time. The inevitable opening of stomata during day time also promote water loss via transpiration. Cactus reverse this process (see Physiology). Other .Xerophytes reduced water loss by having thick epidermal layers or bark (periderm) and or having thick layers of cuticle ( cutin - a waxy substance in the cuticle is impervious to water ). In this way, internal water pressure is maintained. A very important physiologic tactic in order to maintain turgor pressure of the photosynthesizing cells under water stressed condition. The dense shallow root system allows Cactus to absorbed rain water at a very rapid phase before the sandy soil dries up. Except for leaf rolling or leaf folding, Cactus employs these varieties of tactics.
Types of Cactus
Hedgehog cactus. Thick upright stems forming a mound. Ex. Echinocereus spp.
Barrel cactus. Single fat stem that is globular or columnar. Ex. Ferocactus spp.
Cluster cactus. Cluster of balls-small round stems forming a large clump. Ex. Rebutia spp.
Columnar cactus. Thin or thick upright tall stems branching at the base or sides of main trunk. Ex. Saguaro cactus
Prickly pear cactus. Branches have round or cylindrical sections that drop off easily and become rooted. Ex.Opuntia spp.
Sub-surface cacti. The body is buried under the surface of the soil with only the top is exposed. Ex. Gymnocadycium sp.
Epiphy cactus. Epiphytic, rubbery, flexible stems that hang from tres in rain forest. Ex. Epiphyllum sp.
Misc. cacti. Cacti that don't fit in any other category. Ex. Paniecereus sp, Pereskia sp, Brasiliensis sp.
Variations in plant body of Cactus occurs from the smallest, inconspicuous Blospeldia Cactus, attaining 1cm diameter at maturity to a more imposing Saguaro cactus (see photo).The largest among the Cacti is Saguaro that attains the height of about 19 meters. The plant body of a normal plant is composed of leaves, stem, and root as the main organs; while flowers, fruits and seeds as accessory organs.In Cactus, the leaves are modified into a spine (see photo) but normally Cactus are leafless. The stem lacks secondary growth, therefore, no woody tissues were formed since cambial growth does not proceed beyond primary growth. However, Cactus remained fleshy and contain lots of water stored in the vacuoles of their cells thus, maintaining their form and shape despite the limited water supply. The photo shows various forms and shape of Cactus.
Although, Cactus belongs to the same group of dicot plants, Cactus root system follows that of a monocot, a fibrous root system common among grasses, very dense shallow roots, and few primary roots. This architectural designed augurs well in the desert in order to exploit sudden rain showers in the sandy soil.
Leaves are the main photosynthetic organ of a normal plant. but in Cactus, the leaves are modified into the spine with few exceptions, spines are non-photosynthetic its main function is more of protection against browsing animals rather than.photosynthesis
The stems, aside from support and conduction of water and minerals from the roots to the plant body, in Cactus, is the main photosynthetic organ. The stem of Cactus is either ribbed or fluted and inter- spaced with numerous areoles( meristematic region) located along the length of the succulent stem. Areoles forms a rosette pattern in Cactus stem similar to nodes and inter- nodes of normal stems. The Table presents different types of Cactus and descriptions including sample species.
Physiologically; the stem is the main photosynthetic organ of Cactus. The Cactus stem contains chloroplast- the cell organelle responsible for photosynthesis -a food manufacturing process in plants. Cactus is a CAM (Crassulacean Acid Metabolism) plant, unique to these groups is the temporal separation of assimilating and fixing CO2. Carbon dioxide is fixed during night time when the temperature is cooler in the form of organic acid (malate or malic). At daytime. Malate is re- converted to simple forms and the concomitant released of trapped CO2 to participate in photosynthesis when the stomates are closed. This physiologic tactic (CO2 pump) allows Cactus to reduced transpiration and prevent water loss. At high temperature, Cactus remained normal because of high water content acting as the coolant that tends to enhanced thermal tolerance and cellular integrity. The closing of stomates during water stress is a valuable strategy to conserve water. Another adaptation is the presence of cuticle- a waxy substance that covers the epidermal cells of the stems, the main function is to prevent the escape of internal water.
The presence of spines increases reflectance index in Cactus thus increases cooling of cells and water conservation. In other words, in non-xeric species, thermal death is attained at the temperature range of 47-55º centigrade but Cactus tend to tolerate high temperature.
Growth and Development
Growth and development in Cactus are quite limited based on the fact that secondary growth is absent, however, the Cactus stem is ribbed or fluted designed to expand and contract in the presence or absence of water respectively.This anatomical modification provided by the rib meristem allows Cactus to attain considerable height and girth. Additional tissues are made from the cambial growth at the base of Areoles. This meristematic tissue served as the axil of the stem where a branch or petiole of a leaf may develop in normal plants. Moreover, growth and development in Cactus are quite restricted to the stem that tended to form into barrel-shaped or tree -like as in the case of Saguaro Cactus. The increase in height or girth in Cactus is mainly due to the meristematic activity of the Areoles and rib meristem where spines developed to a rosette pattern of short nodes and internodes (see photo).
It's typical in Angiosperms to produce flowers, fruits, and seeds in order to perpetuate their species generation after generation. In Cactus, flowers usually bloom at night and are pollinated by bats and other nocturnal visitors. The flowers come in a variety of colors. Petals are either red, yellow or white. Pollinators like bats are attracted not to the colors of the flowers but because of the chemical it exudes in the air that served as an attractant while moths are drawn to the white colored petals. In Cactus, floral parts are modified, for instance, sepals, and petals are fused together called tepal perhaps to save unnecessary energy in the development of these organs.
The flower that blooms in the evening usually withers at daytime .The pollinated stigma is penetrated by the pollen tube to deliver the male gamete and fertilize the egg inside the ovule, further development shows that the fertilized egg developed into an embryo complete with genetic information to start a new life once detached from the mother plant .Therefore, a matured ovary becomes the fruit while the matured ovule becomes the seed. Cacti fruits come in various colors and sizes so with the seeds. Cactus are highly cross-pollinated, meaning pollen grains comes from a different plant.
Many Cactus fruits are edible, eaten raw or mixed with other fruits (salad).Old age practice of some tribes uses cactus as medicine, in culture rituals as hallucinogens and or spiritual preference. Young Cactus stem is mixed with vegetables while juice extracts are used in the preparation of cold drinks , beverages, and even health products.
In conclusion, plants like Cactus are truly the super plants of the waterless environment. Developed excellent anatomical, morphological, physiological and reproductive adaptations to the hot waterless habitat of the desert and sustain life even in a very limited water supply as aerial Epiphytes. Today, Cactus are grown worldwide for their medicinal, ecological, aesthetic, and economic value.
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