Cycas circinnalis
Cycas circinnalis (Cycadaceae family) is a rare plant native to southern India, Sri Lanka, Java, Sumatra, Taiwan and the Philippines.
Cycas plants have a single, aerial stem that can reach more than 10 metres tall. These plants grow about 1 cm a year. The male and female reproductive organs are borne on separate plants.
The inner stem of Cycas circinalis is consumed in India and Sri Lanka. In Japan, the inner stem of Cycas revoluta is eaten.
Until the end of World War II, Cycas circinnalis seeds were a food source for the Chamarro people of Guam and Rota, islands in the Mariana Islands chain in Micronesia. A number of researchers have found that eating Cycas circinnalis seeds appears to be related to an increased incidence of Parkinson’s Disease associated with progressive Alzheimer-type dementia amongst the Chamorro people. Before 1955, this disease was 50 to 100 times more common on Guam than in industrialized nations.
There is a male Cycas circinnalis specimen in the Hacienda greenhouse.
The male cone sits in the centre of a rosette of large pinnate leaves.
Dioon edule and Dioon purpusii
The name Dioon (Zamiaceae family) comes from the Greek word dis, or two, and oon, or eggs, referring to the pairs of seeds.
Members of the Dioon genus have a crown of persistent, stiff leaves atop a trunk varying in length.
The reproductive organs, in the centre of the plant, are contained in cones. Male and female cones are borne on separate plants. The seeds are the size of a walnut and have a high starch content. They are toxic, however, if eaten raw.
Dioon purpusii is a rare plant native to Mexico.
Visitors can admire some lovely specimens of this plant in the desert regions and Hacienda greenhouses.
Encephalartos
Encephalartos (family Zamiaceae) have a single underground or aerial stem, which in some cases can grow to 10 metres or more. Species with underground stems produce tuberous roots that are thought to be a climatic adaptation, since these plants do not have a stem capable of storing water. Encephalartos plants form aerial roots that contain a blue algae called Anabaena. This symbiotic association allows the plants to capture nitrogen directly from the air.
The pinnate leaves are composed of a main rachis bearing several leaflets with parallel veins, the shape, edges and colour of which vary from one species to the next. These features help to identify the different species, in fact. In mature plants, the leaves appear to be arranged in a whorl, with the number of leaves per whorl varying from 4 to 50, depending on the species.
Encephalartos reproduction and propagation
Encephalartos are dioecious, meaning that the male and female reproductive organs are borne on two separate plants. Sexual maturity is attained only after 25 to 30 years of growth. The cones are made up of a large number of modified leaves (megasporophylls), arranged spirally around a central axis. Each megasporophyll has two ovules varying in size and colour depending on the species. The ovule integument is made up of three distinct parts: a fleshy outer layer, usually orange or yellow, a sclerified middle layer and a fleshy inner layer that dries and quickly takes on a paper-like texture. The male cones are smaller and more numerous than the female cones. There are hundreds of microsporangia, or pollen sacs containing pollen grains, on the abaxial (lower) surface of the microsporophylls.
In the wild, plants have to be close enough together for pollination to occur. The large amount of pollen produced is an adaptation to anemophily, or wind pollination. Some botanists doubt that insects play a role in pollinating Encephalartos. They believe that, even if bees can be observed collecting pollen from male cones, they do not visit female cones. Other botanists think that a weevil from the Curculionidae family, a kind of beetle that lives on Encephalartos, is a pollinating agent. In horticulture, it is especially difficult to collect seeds because the male and female plants, if one has both, must produce cones at the same time.
At pollination time, the ovule secretes a liquid, or pollen droplet, from the micropyle, to capture the pollen. Among Gymnosperms in general, and Cycads in particular, several months elapse between pollination (when the pollen is carried to the ovule) and actual fertilization (when the male and female gametes fuse).
Afterward, Encephalartos development is fairly direct. In fact, there is no dormancy period in Cycads. As soon as fertilization is complete, the development process begins, and once the cone splits and releases its seeds, the well-developed embryo inside the seed feeds directly on the endosperm. If the protective inner layer is broken, the embryo could be damaged by moisture or fungal spores, or heat and dehydration. A waiting period of up to 6 months is recommended before planting, by which time the radicels will have begun piercing the envelope. After 6 or 7 months, the Encephalartos embryo is composed of two cotyledons separated by the plumular leaf and the radicel. Plantlets have less water storage than mature plants and are thus more sensitive to irregular watering.
There are also a number of vegetative propagation methods. The first, obviously, is to separate the suckers that form at the base of the trunk, with their own roots. This must be done carefully, and fungicide applied to the parts injured by the incision. Young plants formed if the main trunk is damaged near the top may also be separated. Such ramifications are infrequent and are often confused with the plantlets that may form from the crown of the parent plant. This unstable situation can last only as long as the plant is not disturbed. Obviously, it can happen only with female plants.
Stangeria
Stangeria eriopus is related to the Zamiaceae but belongs to the Stangeriaceae. It is a rare plant native to South Africa, where it grows in coastal grasslands and inland forests.
Four pinnate leaves grow from the tip of the underground stem. In forest forms, the stem may be up to 2 metres long.
The male and female cones are borne on separate plants. Each stem produces a single cone.
The Garden’s collections contain one male and one female plant.
Zamia
Zamia floridana (family Zamiaceae) was once fairly common throughout the Florida peninsula. Today it is a rare species, limited to pine groves, dry sites and coastal dunes. Most of its habitats have been destroyed, in part by the tourism industry.
The male and female cones are borne on separate plants.
Zamia floridana rhizomes contain a toxin that must be destroyed before their starch is extracted. Many soldiers are said to have died during the American Civil War (1861-65) from eating uncooked rhizomes. The toxin is also present in the leaves and seeds. It causes tremors in livestock. Once properly processed, the starch extracted from the rhizomes can be made into flour. This plant was a major food source for local inhabitants in the 19th century.
Based on articles by Céline Arseneault and by Denis Barabé published in Quatre-Temps magazine.
