The Century
Plant
A spectacular succulent which flowers once, then dies, has erupted into flower in the Royal Tasmanian Botanical Gardens. This Century Plant (agave americana) near the Cactus House, has flowered once in its 10-year life, however some Century Plants take 70 years to flower.
With the plant stem an impressive six metres high, the rare vivid-yellow floral display is a sight to behold. The yellow flowers massed along the stem will never be seen again on this plant.
A huge amount of energy is expended by the plant to produce such a large flowering stem, which means that after this flowering, the plant will die. Originating from Mexico, where it can reach a height of up to 12 metres, the Century Plant is monocarpic - which means it flowers just once and then dies. This plant has an offshoot, so it will be reborn to flower once again.
Photo: Beth and The Century Plant in the Royal Tasmanian Botanical Gardens - Jan '99.
Cactus or Succulent?
Any plant with the ability to store water in its leaves, stems or roots can be called a succulent. Succulents can be divided into two main groups.
Stem Succulents (no fleshy leaves) - This type is typified by the cacti (Cactaceae), the spurges (Euphorbiaceae), and the milk weeds (Asclepiadaceae)
Leaf Succulents (fleshy leaves present) - Included in the group are the crassulas and sedums (Crassulaceae), the "living stones" (Aizoaceae), the aloes and haworthias (Liliaceae) and agaves (Agavaceae)
Although the cacti are in fact a type of succulent, they are so distinctive they are generally classed as a separate group.
Specialised Survivors
Succulents display dramatic adaptations in response to living in some of the harshest environments on the planet. They dominate and thrive in areas where other plants shrivel and die due to exposure to sunlight, severe contrasts in temperature and lack of water. Here are some of the amazing characteristics displayed by succulents. Can you spot them in the Cactus House?
Save it From a Rainy Day
The most obvious adaptations the succulents have is the ability to store water in juicy leaves and stems, enabling them to survive long baking dry spells. If the dry period is extended, the plants will dry and shrivel, only to pump themselves up again when moisture is available.
Water storage would be of little benefit if the plants sported luxuriant foliage, as moisture would be quickly lost by evaporation. To avoid this, many succulents have reduced their leaves and stems to a minimum, some to such an extent their leaves have been replaced by the familiar spines of the cacti.
Others drop what leaves they have during times of drought, thus minimising the loss of precious moisture.
A Shady Side
The succulents have also developed many different ways of creating their own shade from the baking sun. A spherical shape ensures that part of the plant is in the shade at any time of the day. However it does expose the most delicate part of the plant, the growing crown, to the sun, so clusters of spines are often present on the crown to create a shade umbrella.
Many species are deeply ribbed, thereby creating shaded Crevices on their surfaces. The crevices are sometimes multiplied towards the crown. Other species have developed a woolly or spider web coating to keep the sun off. The wool is a variation of the more familiar spines.
Night Time Breathers
Plants produce food by combining carbon dioxide, water and sunlight. Most plants are busiest collecting carbon dioxide during the day when sunlight is available, through small pores called stomata. While the stomata are open, however, the plant loses moisture by evaporation. To avoid this, the succulents only open the stomata in the cold of the night and so effectively are holding their breath during the hottest part of the day.
However, photosynthesis can only occur during sunlit hours, so they produce organic acids from the waste products of their own growth enabling the combination of the carbon, collected at night, with the energy of the sun during the day. This process is known as Crassulacean Acid Metabolism (C.A.M.)
When it Rains
The root systems of succulents are also highly specialised to take advantage of an);moisture that comes their way. Some penetrate deep into sandy soil to reach receding water supplies deep below the surface. Others spread their roots just below the surface to collect surface moisture such as early morning dew.
Keeping Hungry Jaws Away
The spines of the cacti are an effective deterrent for any animals seeking a moist meal to sooth their parched palates. Other plants have developed skins too tough to chew.
A fascinating defence mechanism has been developed by the "living stone" (Lithops sp.) of South Africa. Their camouflage is so effective that they are almost identical to the stones they grow amongst, and are simply overlooked by herbivores.
During the rainy season the "living stones" become obvious, because like most plants they must flower to produce seeds, but the deserts are so alive with edible vegetation at this time that the deception is no longer vital.
Floral Displays
Succulents produce a wide variety of floral displays, from the brilliant torches of the aloes in winter to the spectacular blossoms of the cacti in summer. Some flower at an early age, as in the mammillaria; others, such as the Century Plants (Agaves), may take up to 70 years to flower.
Growing Succulents
Most succulents are easy to keep, as long as they are not overwatered, especially during winter. In fact, even in summer they can be left without watering for a week or two without harm. They require lots of sun and are quite happy on a sunny window sill. Many will grow outside, but protection during winter, such as a porch or glasshouse, is recommended. Succulents can suffer from a few pests such as scale and mealy-bug. These can be eradicated with prudent spraying. Contact your local cactus club or nurseryman for advice.
Pamphlet prepared by Andrew Smith, Education Officer, Royal Tasmanian Botanical Gardens
Copyright RTBG 1990