|Posted by: Fungusmaximus Jul 29 03, 03:19 PM GMT|
The stems of cacti are thick and succulent. The soft, succulent tissues of the cortex and pith provide a large volume for storing food materials and water. A thick waxy cuticle covers the epidermis and restricts loss of water.
The stems may be simple columns with no side shoots or may be branched either near ground level or far above it. The stem surface may be smooth, but is most commonly covered with tubercles, or rows of them, more or less connected, called ribs, which represent the sublimation of all branches. Each tubercle bears an areole, a minute hump of tissue in which spines are propagated. The buds of areoles can eventually differentiate into flowers and then fruit depending on the presence or absence of appropriate environmental stimuli and particular precursors. Areoles are found only in cacti. Removal of spines causes only superficial damage.
Spines develop from the axillary buds and can be considered to be modified leaves. The spines are always located on the areole. In most cases two forms of spines can be distinguished: central spines that are located close to the apex of the areole, and radial spines which grow around the circumference of the areole apex.
In addition, specialized spine forms are often characteristic of individual species. Spines that are curved into a hook are found in many species of the Mammillarias. Cacti with such spines are commonly shallow rooted. The combination of a shallow root system and hooked spines facilitates dispersal. A plant that is inadvertently ripped from the ground by large animals passing by attaches to fur via the spines; the plant is eventually discarded and can readily take root.
Members of the sub-family Opuntioideae possess distinctive hair-like spines called glochids. Glochids are only a few millimeters in length. They are thin and brittle and have fine barbs, which can only be seen with the aid of a microscope. Glochids easily pierce the skin and then break off, The barbs help to push them deeper into the wound, resulting in a great annoyance for several days. The infamous itching powders are derived from glochids.
Spines have important benefits for cacti. The central spines are commonly long, strong, rigid and brightly colored and they deter grazers. (The grazers soon learn to stay away from brightly colored spines). The radial spines are thin , flexible and are often white. White spines reflect sunlight away from the plant, (and all spines provide shade) reducing the potential for damage from excess solar radiation. Some species, e.g. Espostoa lanata, on the very arid western slopes of the Andes, capture and absorb daily ocean mists on the ample woolly spines. The down-pointed central spines drip any excess droplets to the shallow roots in the soil below. During a light rain or mist, water accumulates and condenses in droplets on the tip of each spine. Downward directed spines cause the water to be deposited on the soil surface, just above the shallow roots.
Some cacti are spineless or nearly so. These tend to produce toxic substances that compensate for the lack of protection from grazers offered by spines. Lophophora williamsii (Peyote) is one such example; it produces the alkaloid, mescaline, which is hallucinogenic and is used as a narcotic legally only by one "Native American" religious group. All other users do so illegally and some states prohibit even hobby cultivation of Peyote. Epiphytic cacti, the "Jungle Cacti" (Holiday, Orchid and Mistletoe cacti) are also nearly all spineless. These grow on trees and on cliff faces, habitats mostly inaccessible to predators who would be repelled by spines. These are treated more fully in later sections.
The Root System
Commonly, the root system of cacti is very shallow and widely spread, which enables them to exploit water deposited in surface horizons by short periods of rain. For example, the root system of a young saguaro (Carnegiea gigantea),which was only 12 cm high filled an area of about 2 meters in diameter but penetrated only 10 cm into the soil (Benson, 1982).
In addition to their shallow, branching roots, many columnar cacti, including the saguaro have one or more taproots that penetrate the deeper layers of the soil. These taproots help to anchor the plant and to obtain deeper lying water and nutrients. In some cacti, e.g. the genera Ariocarpus and Lophophora, the roots actively shrink during periods of drought. In the process, the shoot is drawn into soil and the entire plant may become covered by soil. This conserves valuable water by reducing the surface area of the plant exposed to the air, and protects them from grazing predators. Such plants have been called "living roots".
The cactus flower is similar to those of other flowering plants. It is a perfect flower, which means that it bears sepals, petals and functional pistils and stamens. Most cacti have numerous stamens. All cacti, except for some species of the primitive Pereskia, have epigynous flowers. The flowers of some primitive Pereskia species are perigynous, which differ from the epigynous flowers of all other cacti in that the ovary is superior rather than inferior.
Cactus flowers vary in size and color. The flowers of Selenicerus grandiflorus(Queen of the Night) can be more than 20 cm in diameter (Benson, 1982). In contrast, the flowers of Mammillaria are commonly very small but cluster together to form a conspicuous garland just back from the crown of the plant. Colorful blooms and scents attract pollinators. Many desert cacti produce colorful unscented flowers that bloom during the day and attract various flying insects. Many jungle cacti have richly perfumed large blooms, usually white or pale yellow, that open at dusk and attract moths. Night-blooming cacti tend to have large, foul smelling, fleshy blooms that attract bats.
To survive in very dry and hot places, cacti must be capable of
- surviving long periods without water
- being very economical with any available water
- tolerating intense solar radiation and high temperatures
The following features contribute to these capabilities:
...Cacti are very slow growing.
Consequently, their water requirements per unit time are low.
...Spongy tissue: Cacti have special water-storing tissue.
The majority of epidermal and inner stem tissue consists of water-storing parenchymatous cells.
...Cacti exhibit Crassulacean Acid Metabolism (CAM).
This results in high WUE (water use efficiency). WUE refers to the amount of water lost by transpiration relative to the amount of carbon dioxide taken up and fixed into organic compounds by photosynthesis.
Most plants open their stomata during the day in order to allow carbon dioxide to diffuse into the leaf and, with water, to function immediately as a substrate for photosynthesis. Inevitably, this also allows water saturated air surrounding the photosynthetic tissue to diffuse outwards; at night, when photosynthesis ceases, the stomata close in order to stop the loss of water.
CAM plants have a special mechanism that allows them to take in carbon dioxide at night and hold it in organic acids; this causes the pH to fall at night (hence the name acid metabolism).
During the daytime, the carbon dioxide is released from the organic acids and feeds into photosynthesis; stomata remain closed. Opening the stomata at night, rather than during the day results in much less loss of water because of lower air and tissue temperatures. It has been calculated that cacti lose 1/6000th as much water as a mesophyte of the same surface area, by such strategies.
...Cacti are able to survive after considerable water loss.
Cacti commonly lose 60% of their water before significant physiological stress sets in. Most mesophytes, in contrast, will wilt and die with a 20% loss of their water (Benson, 1982).
...Cacti have mechanisms to rapidly seal off wounded tissue.
When a typical mesophyte is cut or wounded its exposed tissue will dry out rapidly. A leaf or branch of a wounded plant may wilt or die unless enough water can be supplied by the root system to compensate for the rapid water loss.
In arid environments, cacti cannot rely on rainfall or moisture in the air or soil to compensate for a loss of water through a wound. However, cacti produce a special, slimy polysaccharide which will cover a wound and dry into a tough, impermeable layer preventing further water loss. Another mechanism of sealing wounds is the production of a layer of water-resistant cork around the wounded tissue. This mechanism is common in the Saguaro.
...Cacti have mechanisms to reduce exposure to sunlight.
Reduced exposure may be needed to reduce overheating, or excess photoxidation. Light colors reflect more of the sunlight away from the plant. The waxy coatings of many species are often light or even white in color. Many columnar cacti such as Cephalocereus senilis (Old Man cactus) have snow-white hairs covering the entire body. A major function of radial spines is to provide shade for the plant.
Water-losses and over-heating is also reduced in low profile cacti that have stems that are retractable into the soil, growing half-buried in the ground and/or being pulled underground by shrinking, napiform roots in dry times.
...Cold-adapted Cacti apply 'anti-freeze' strategies.
Glutinous sap, dark, heat-absorbent colouration, sparse spination, are the main enhancers of internal temperature of plants adapted to high altitutes or high latitudes. *Trichocereus pachanoi exhibits all of these adaptations.
Reproduction and Pollination Ecology
Cacti reproduce both sexually and asexually.
In asexual reproduction all new plants are genetically identical to their parent plants. Asexual reproduction can occur naturally from detached joints, offsets, cell propagation and cuttings, including scions in grafting which produce roots after they have separated from the parent plant. Joints separated from the parent plant which do not develop roots eventually die.
Sexual reproduction is associated with the processes of flower formation, pollination, seed production, dispersal and germination. Plants can be self-pollinated if pollination occurs within the same flower or a different flower of the same plant. Cross-pollination occurs when pollen grains are transferred to a flower of another plant.
Most cacti ensure cross-fertilization in two important ways. First, most cacti are protandrous, meaning that the anthers shed pollen before the surface of the stigma is receptive. Secondly, most cacti are self-sterile, which prevents self-pollination.
Cacti accomplish pollination in a variety of ways. The conspicuous and variable cactus flower color, form and scent serve to attract pollinators such as birds, insects and bats to the food supplies that the flower offers. Pollinators disseminate pollen grains, and fertilization is encouraged. The different forms of the cactus flower often reflect the differences in the body structure of the pollinators that are adapted to fertilize them.
Pollinators frequent the flowers because of the food they offer, the pollen and nectar produced by the flower. Pollen is rich in both fats and protein and is eagerly sought out by bats and beetles. Many cactus species, such as the Saguaro (Carnegiea gigantea) will produce large quantities of pollen. A single flower of this cactus plant may contain over 3000 anthers and subsequently produce hundreds of thousands of individual pollen grains.
Nectar is a watery solution of sugar, secreted at the base of the flower, that is consumed by many insects and birds. Cacti in many genera have developed elaborate tissues for the production of nectar.
Insects, birds and bats are important cactus pollinators.
The most important cactus flower pollinators are the Hymenoptera (bees and wasps), the Lepidoptera (butterflies and moths), and the Coleoptera, (beetles).
Most insects have a well developed sense of sight and smell due to their antennae and faceted eyes. These insects prefer the rotate, raylike flower shape that most cacti possess.
Epiphyllum oxypetalum (Dutchman's pipe) is specialized to suit certain nocturnal hawk moths. This cactus has a floral tube that is exactly 30 cm long, with nectar at its base and it flowers only at night. Attracted by the flower's scent, the hawk moth hovers close to the flower, unrolls its proboscis that is also 30cm long and inserts it into the tube.
Birds form the largest group of cactus pollinators next to insects. The bright, variable shades of red, yellow, or color combinations of red and yellow, red and green and red and blue serve to attract specific birds.
There are approximately 2000 species of birds capable of approaching and pollinating cactus flowers. The hummingbirds are the single most important group to visit the cactus flowers. They have poor sense of smell; flowers that attract these birds are brightly colored, generally scentless and possess stout floral tubes that are suited to their beaks and long tongues.
Schlumbergeras, the Christmas and Thanksgiving Cacti are well adapted to hummingbird pollinators. The dimensions of the flowers are apated so that their long-projecting filaments allow pollen to be easily dusted off onto the head of the bird. When the humming-bird flies to another flower, the pollen is readily brushed off onto its stigmas.
Bat pollination is common for some species of cacti that have relatively large, funnelform, white and pungent-smelling flowers that produce large amounts of nectar and pollen.
The most important nectar-feeding bats are the nocturnal, long-tongued vampires Glossophagidae. They are highly specialized to visit the flowers of many columnar cacti such as the Saguaro and the Organ Pipe cactus. The bats either cling to the large waxy blooms or remain more or less in fluttering flight, extracting large quantities of nectar and pollen by rapidly extending and retracting their long tongues from the flower.
|Posted by: Fungusmaximus Jul 29 03, 04:56 PM GMT|
| Cacti are...
Cacti are succulent xerophytes. Succulents are plants that store large quantities of water in their leaves, stems, or roots, giving them a fleshy (succulent) character. Many succulent plants grow in drought-prone, nearly arid climates or physiologically dry soil (e.g. frequently frozen or partially or periodically salty soils). This involves adaptations that enable them to make do with a minimal amount of available water, and to survive long periods without rainfall. In botanical ecology, plants with these qualities are called xeroph ytes.
Terrestrial plants have three organ systems: the shoots, the roots and the leaves. In cacti however, these systems have undergone extreme evolutionary modification as a means of adaptation to droughty conditions. Leaves have been sublimated to spines (to reduce water loss), stems evolved succulence to store water and nutrients; and they gained chlorophyll to supplant the leaves in photosynthesis. Typically, the root systems are shallow and wide-spreading to provide maximum access to water from dew, mists or sparse rainfall.
Cacti are members of the family Cactaceae.
Cacti are perhaps the best known family of the succulents. Other succulents are often mistaken for cacti, based on their similarity of form, but have no direct relationship to cacti. These other succulents belong to any one of about 37 other families. The Cactaceae belong to the class Spermatophyta (seed-bearing plants) and the superfamily Angiospermae (flowering plants).
Cacti are almost exclusively natives of the Americas.
Their point of origin is thought to be Southern Mexico or Northern South America (Nobel, 1984) where they are most numerous, but cacti are also found in central and western Canada, from the Peace River district, Lat. 57N (Opuntia, Coryphantha) increasing in number of species and varieties of form southward through Central America and the Gulf Islands to Mexico. Here, they continue southward, declining in numbers and forms, finally petering out in Patagonia, at Lat. 49S (Opuntia). Only exemplars of the gen us Opuntia can be found in all cactus habitats of the Americas. Only one species, a Pereskia, has been long known in West Africa. Almost certainly it was introduced there, presumably by ocean currents or more recently, perhaps, by mariners. Cacti are also endemic in the Galapagos Islands off the west coast of South America.
Cacti share 7 defining traits.
If a plant lacks any one of the following traits, it is not a cactus.
Cacti are dicotyledons. Their seeds always produce two cotyledons or seed leaves upon germination.
Cacti are perennial shrubs or herbs, which live on year after year. Annual species do not exist.
Cactus flowers usually have an indefinite number of sepals and petals and have numerous stamens.
Almost all cactus plants, except for the primitive Pereskia have an epigynous ovary.
The cactus fruit is a one-celled berry, smooth or spiny, with seeds scattered throughout.
Cacti are caulocarpic: that is, they do not die after flowering.
Cacti bear areoles, a unique cushion-like structure on the stems which bear spines or flowers.
Five Common Fallacies
(Adapted from Chidamian, 1958)
All succulents are cacti.
Nonsense! About 37 plant families contain succulent plants. Only true cacti form the unique family Cactaceae..
All succulents grow in full, blazing sunlight.
Not so! Very many xerophytes including some cacti require the shade of shrubs, grasses, rocks, etc. to avoid harmful scorch and dessication. Many cacti of the harsher habitats protect themselves with dense spination (Espostoa), very dark colours (some Lobivias), very low profiles above ground (Ariocarpus, Lophophora) and other strategies.
All succulents grow in the desert.
Although the deserts of the world may contain the largest number and variety of cacti and other succulents, they occur also in alpine, jungle and shoreline habitats. Cacti thrive in the low-lying and warm valleys and plains of Central America, in the full blaze of tropical sun, or they can inhabit the rocky and arid slopes of the high mountain systems of the Andes in Ecuador, Peru and Bolivia, where extremes of sullen heat and bitter cold are common. Certain species of Opuntia and Mammillaria are covered with snow in winter, in the arid or semi-arid districts of Arizona, Utah and Colorado. Others are epiphytic, living on the mossy trunks of trees in tropical or sub-tropical forests or on ledges of rocks in deep ravines where they receive ample shade, warmth and a moist soil rich in humus.
All succulents can grow in pure sand.
Nothing grows in pure sand! Cacti and other succulents are adapted to a great many different but nutritious environments and soils. Desert sands are often nutrient rich.
Succulents do not need water.
Water is absolutely essential to all life! Although most cacti and other succulents require much less water than other plant families, they must have a sufficient supply of water in order to support life. For example, succulent plants do not e xist in very extreme deserts such as the Sahara or Australian outback, where conditions are especially arid.
|Posted by: ion Jul 29 03, 09:14 PM GMT|
| I'm going to try to go through this thread and highlight points of special interest. I will try to add more stuff about alkaloid production, soon.
|Posted by: Fungusmaximus Jul 31 03, 08:00 AM GMT|
| I have more good info I just have to get all the hyperlinks active. a mega ton of pics and descriptions of a large number of species.
Its gonna be great to have handy.
I found all this info in search of an illustrated diagram of the anatomy of cactus specifically trichocerus.
Ion, will you aid me in labeling all the parts of the cactus including the interior of the cactus, so I can make a nice photo/drawing illustration for the board??
I sometimes wonder what the hell something is and don't know how to refer to it. THis will aid us cactus newbies
|Posted by: ion Jul 31 03, 04:08 PM GMT|
|No problem. Just gimme a picture to color on!|
|Posted by: Fungusmaximus Jul 31 03, 05:39 PM GMT|
|Posted by: ion Aug 11 03, 03:58 AM GMT|
| Biology and Phytochemistry
The cacti, and other succulents like the stonecrops (family Crassulaceae), utilize a method of carbon dioxide fixation within their photosynthetic parenchyma cells known as crassulacean acid metabolism (CAM). All other plants photosynthesize by directly exchanging the necessary gas, carbon dioxide, with the environment through their diurnally open stomates (tiny apertures on leaf surfaces that exchange water and gas with the environment). Because of the dry, usually hot environment where they live, CAM plants are forced to close their stomates during the day to prevent water from evaporating. (Since the molecular weights of CO2 (44g/mol) and H20 (18g/mol) differ significantly, water diffuses much faster across a gradient, and would be lost at a rate that would kill certain plants (i.e., succulents) that do not have abundant reserves). With stomates that are closed during the day, no water or CO2 is exchanged and a new mechanism is required for productive photosynthesis. CAM plants, instead, open their stomates and fix CO2 within their cells at night, and then rely on thermodynamic chemical reactions to store and convert the CO2 for use during the day. The ability to fix carbon dioxide during dark hours is due to the activity of the enzyme, PEP carboxylase (occurring in the cytosol). This enzyme converts the nocturnally attained CO2 to an organic acid (such as malic or isochloric acid) for storage in the cell's vacuole until the morning. With the heat energy of the day, the equilibria within the plant's cells are shifted, and the organic acid product of nighttime CO2 uptake is then decarboxylated into carbon dioxide. This chemistry is why CAM plants taste sour at night and sweet during the day. After this thermoperiodic process, the CO2 is then transferred to the five-carbon sugar, ribulose 1,5 bisphosphate, beginning the Calvin cycle. Here, the process of photosynthesis begins within the same cell that generated the CO2, making for a very efficient system of growth and development. Warm nights, however, cause the organic acids to be converted to useless CO2 because no photosynthesis can occur without light. This is the reason why many cacti are so slow growing.
At night (left), CO2 is fixed via open stomates while starch from the chloroplast is broken down to phosphenolpyruvate (PEP). The CO2 undergoes hydration to from the bicorbonate ion (HCO3-), which reacts with PEP forming oxaloacetate. Oxaloacetate is reduced to form malate, and then pumped into the vacuole. There, it is stored as malic acid until the morning.
When the plant cell is heated by the sun (right), the vacuole expels malic acid which is then decarboxylated. The products of this reaction, CO2 and pyruvate, act on the chloroplasts. CO2 is fixed by Rubisco in the Calvin cycle, and pyruvate undergoes reverse glycolysis to form starches and sugars.
|Posted by: ion Aug 11 03, 04:00 AM GMT|
| The phytochemistry of the cacti is also distinct. They contain rare primary compounds and unique secondary metabolites. First, the pigment compounds known as betacyanins (or betalains) are found in only a few orders of flowering plants. All other angiosperms contain either flavanoids, flavonols, or anthocyanins. Betacyanins are reddish compounds of the fruits and flowers, which are similar to anthocyanins but more complex due to their aromatic (nitrogen-containing) constituents. Other than in the cacti, betacyanins are found in the Chenopodiaceae (goosefoot and beet) and Portulacaceae (portulaca) families. The cacti also contain alkaloids, small organic compounds containing heterocyclic nitrogen. These secondary metabolites, derived from amino acids and used to move nitrogen throughout the plant, serve allelopathic functions to deter competing plants and plant eaters. Accumulating over time, they are found in highest potencies in the older plants. Because alkaloids are often bitter-tasting and or poisonous, many herbivores would rather starve than eat plants containing them. Also, since alkaloids are expelled through the plant' s roots, many nearby plants are outcompeted for light and water because they are chemically unable to process the toxins produced by their neighbors. There are, however, certain organisms (usually humans) that can metabolize these compounds and often enjoy them! Caffeine, nicotine, cocaine, and all hallucinogenic drugs contain alkaloids. Mescaline, the most commonly known alkaloid produced by the cacti, has been used in traditional religious settings by North and South American Indian tribes for thousands of years. It and other recreational and medicinal cacti will be discussed in the next section.
The previous 2 posts were of info taken from: http://www.nybg.org/bsci/herb/cactaceae1.html
Highly recommended reading.
|Posted by: Archaea Aug 11 03, 06:06 PM GMT|
| I have that work!
It is interesting to note that some SW texas acacias have been found to contain some of the same alkaloids previously thought to be found in cacti alone.
The cactus family is closest to the Purslane family I believe so the pigment thing makes sense.
The best info I have found on the biochemical pathways for Mesc is from the tyrosine and adrenaline pathways in mammals. Anyone have more luck?