The Genus Aloe
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C O N C A L S E R N I V A A T O T R O Y B U S N I I V V E A R D S I I A T Y R N O F C A L I F O volume 1, issue 1.0 university of california davis botanical conservatory July, 2009 The Genus Aloe Aloe has a long ethnobotanical and medicinal history or form stems which around world. Here we will look at aloe from both a may be single, tree- botanical and horticultural viewpoint, and also take like, or branched, a brief look at the facts and the myths surrounding shrub-like. Most its medical uses. stand perfectly upright while a Aloe vera is is a commonly known plant, but it is few creep along only one of about 400 species in the genus Aloe. All the ground. Even aloes have rosettes of fleshy leaves, which may be though they are spined or smooth. The majority of aloes have spines now grown around The distribution of the genus of various rigidity the world for their Aloe is shown in orange. along the edges beautiful forms, of their succulent flowers, and medicinal properties, aloes originated leaves. The flowers around the African continent. They are native to are tubular shaped, sub-Saharan Africa, the Saudi Arabian Peninsula, and come in colors and to many islands of the western Indian Ocean, ranging from near- including Madagascar. The center of diversity for white to yellow to aloes is South Africa, which hosts more than 120 orange to near-red. species and has a rich heratige the plants. The flowers are held high on single or branched stalks, and the resulting seeds are held in dry capsules. Pollination of aloes in their native range of Africa and adjoining areas is done by non-hovering birds. As a result of this co- evolution, there is always a rigid stalk for the birds to take hold of while they probe flowers for nectar. The range of sizes among the species varies greatly, from small grass-like aloes such as A. descoingsii, whose rosettes can be less than an inch wide, to large tree One of the most beautiful and collectable of all aloes that can grow to be 50 such as A. bainesii. They aloes is Aloe polyphylla, seen here growing at may be ground hugging and referred to as stemless Hortus Botanicus Nursery near Fort Bragg, Calif. • botanical notes • Aloes are often thought to only grow in hot and day, the amount of water that can escape through dry climates but they actually grow in a variety of the stomata would cause a desert plant to dry out, climates including desert, grassland, and coastal or but it needs to keep these pores open to collect the even alpine locations. One of the most beautiful carbon dioxide needed for photosynthesis. In CAM of all aloes, A. polyphylla, can be found as high photosynthesis, the stomata only open at night. as 11,500 feet above sea level in the Drakensberg This way the plant gets as much carbon dioxide as mountains of Lesotho. In most cases aloes are found possible while minimizing water loss through the growing in rocky or gravelly soils, but they will also stomata. The carbon dioxide is stored overnight as grow in richer soils among grasses, out of cracks and an acid (malic acid) until the next day when sunlight crevasses in rocks, or in sand. Aloes usually grow in provides the cue to release the carbon dioxide which the open and won’t normally be found among shrubs the plant then converts into sugars, the energy or trees unless the sun is very intense, where they use currency of plant cells, to use and store for later other plants for protection. Most often, populations use. Another adaptation is an aloe’s ability to store of aloe will be sparse, although sometimes they are relatively large amounts of water in the leaves, so in the dominant species in an area. In Africa, aloes times of drought they will have a supply to draw are pollinated by sunbirds, whose long beaks and upon. A waxy coating on the surface of the leaves tongues are well suited to extract the nectar from the of aloes also helps with moisture loss by minimizing flowers. In the Americas, hummingbirds pollinate evaporation of water from cells on the surface of the our garden aloes! leaves and limiting water loss to the stomata. The wax can also reflect excess light away from the plant. Aloes have recently been placed in the family This wax can be seen on the leaves of most aloes as Asphodelaceae after years of being considered a blueish color cast. To deter herbivory by thirsty part of the lily family (Liliaceae). Although still desert dwellers, aloes have developed two important related to the lily, aloes and other plants now in features. Most aloes have spines along the margins the Asphodelaceae family have been found to of their leaves, and sometimes throughout the leaf have significant morphological and ecological surface, to help prevent larger animals from chewing differences from the lilies. Both the Liliaceae and on the leaves. For smaller pests and those not Asphodelaceae, along with about 20 other families, deterred by spines, a bitter exudate is produced just are part of the larger order of plants known as under the surface of the leaves. This is known as aloe Asparagales. A distinguishing characteristic of the bitters, a substance that humans have been using for Asparagales is their pigment rich black seeds. medicinal purposes for thousands of years. Aloes are successful because of several important The name aloe is presumed to come from the Arabic adaptations to their environments. They use a special word alloh, meaning shining bitter substance. kind of photosynthesis called CAM (Crassulacean From its leaves two different products are derived: Acid Metabolism) that minimizes water loss that aloe gel, and aloe latex. These two substances vary would occur with standard photosynthesis in hot considerably in their chemical composition and climates. CAM photosynthesis occurs in nearly have been used for medicinal purposes since ancient all other succulent drought adapted plants. In times. Aloe gel, which is found in the interior of the “normal” plants, the pores (called stomata) act as a leaves, has been used as a topical treatment for a vent that takes in carbon dioxide as well as releases variety of skin ailments. The gel works by hydrating water for cooling (transpiration). On a very hot dry and protecting a topical wound until the body can page 2 • botanical notes • repair itself. The bitter latex comes from a layer of Botanical Garden or the Ruth Bancroft Garden in cells just beneath the outer skin and is used to cure Walnut creek. Both of these gardens have beautiful intestinal troubles. This sap is taken internally and displays of succulent plants that can be inspirational. soothes digestive complaints by acting as a purgative The climate of these two gardens is slightly milder or laxative. The real medical benefits of aloe have than that of Davis and some of the aloes will need yet to be proven but at the least many people now more protection in the winter. Many species of believe in the benefits. It is important to note aloes are offered at the plant sales on the UC Davis that some species of aloe are poisonous, so correct campus and other public gardens, or they may be identification of the species must be made before obtained online from specialty nurseries. treatment since only Aloe vera and several others, such as A. ferox in South Africa, are known to be of One of the best reasons to grow aloes is that they medicinal value. require little care. In the summer months they only need to be watered about ever other week, and in Aloe vera, unfortunately, is too cold sensitive to be the winter combination of cool temperatures and grown outdoors successfully during our cold wet rainfall will always provide more than enough water winters. There are many other aloe species, including for them to survive. When transplanting into the the medicinal A. ferox, that can be grown successfully ground make sure the soil is fairly dry, that roots outdoors in the region of Davis, California (Sunset are gently loosened to be surrounded by the soil in zone 14, USDA zone 9). Some are hardier than the planting hole and don’t water for at least one others, but all those described below are worth trying. week afterwards. Fertilize only lightly in mid spring Aloes make beautiful additions to gardens due to and late summer if the soil is sandy or if grown as a their bold architectural forms and vibrantly colored potted plant with balanced liquid fertilizer or slow- flowers. The bluish leaves and rugged appearance release pellets. Pests are rarely a problem, but when contrasts well with more delicate plants, and they they crop up it’s best not to use harsh chemicals on can look especially dramatic planted as a colony or a the succulent leaves. border. They may be included in a perennial garden, a tropical garden, a desert landscape, or featured as a Aloes are tough plants, but they can be tender in the potted plant. The key to growing aloes is simple since wintertime. Many species will survive the winter fast drainage and plenty of light is best for nearly all in Davis, but protection will probably be necessary of them. To provide enough drainage, plant aloes when the temperatures go below 32°F for longer and other succulents on a raised bed or mound or a than a few hours.