Among Cactaceae, the leaves are not large enough to ensure the plant’s survival. Instead, the functions of assimilation and transpiration are transferred to the stem, which performs photosynthesis and stores water.
Cactaceae come in a wide variety of shapes, from spherical to disk-like, cylindrical or spreading.
The stems of several Rhipsalis species are reduced to filament-like structures, while other species in this genus have leaf-like stems. Some cacti, including those in genus Cryptocereus, have stems shaped like fern fronds. There are also some climbing vine-like Hylocereus that grow in South American tropical rainforests.
The Cactaceae family includes 2,000 species in 300 genera.
- Pereskioideae (primitive cacti)
- Opuntioideae (prickly pears)
- Cactoideae (typical cacti)
The first two sub-families are smaller than the Cactoideae, which accounts for about 90% of Cactaceae species, with plants in a tremendous variety of sizes. For instance, an adult Blossfeldia liliputana is only 9 mm in diameter, while Carnegiea gigantea can grow up to 12 m tall.
The Botanical Garden collection
The Garden’s Cactaceae collection contains 132 genera and 700 species.
Among them is genus Echinocactus, whose name is derived from the Greek word echinos, which means “spiny.” One of its English names, mother-in-law’s cushion, refers to the many sturdy spines arranged on its areoles. Echinocactus grusonii is the most spectacular species in this genus – it can grow to 1.3 metres tall and 80 cm across. Unfortunately, its popularity as an ornamental plant has helped reduce native populations. Its yellow flowers need full sun and high temperatures to open.
The Garden has had a Cactaceae collection since it first opened.
In 1937, the Garden received some seeds of Cephalocerus senilis, a rare species that is still grown in its collections. This species is native to the arid, rocky regions of the Mexican states of Guanajuato and Hidalgo. The columnar plants, rarely ramified at the base, can grow to 15 metres tall. They bloom at night, at the top of the plant, only once it has reached 6 metres. Young plants are covered in long, woolly white beard-like hairs, hence this species’ common name of old man cactus. It was long considered a curiosity and this led to over-harvesting, causing it to almost disappear from its natural habitat. It is currently protected by the Mexican government, and only seeds are available to scientific institutions.
Another rare species in our collections is Nopalxochia ackermannii, received in 1938 in the form of seeds. It is native to the subtropical mountains in the Mexican states of Veracruz and Oaxaca, where it grows at altitudes ranging from 1,500 to 2,000 metres. N. ackermannii belongs to the Epiphyllum group, in terms of both its growth habit and flowering. The bright red tubular flowers open only at night, explaining why the Indians considered this a sacred plant. After it was first cultivated in 1832 it became so popular for its lovely blooms that it almost disappeared from its natural habitat.
The genus Agave, belonging to the Agavaceae family, contains 300 New World species with thick, fibrous leaves, most often succulent, arranged in a spiral rosette.
These perennial plants take several years to grow and flower. After growing for 8 to 20 years, at a precise moment, the inflorescence develops from the centre of the rosette of leaves. The flower stalk rises from the rosette to a height of several metres, and dries out once the bulbils (plantlets that will grow into new plants) or seeds (for sexual reproduction) are fully mature. The leaves, arranged in a spiral, are generally thick and succulent. The rosette dies after the plant flowers.
The living stone family, Aizoaceae
The name “living stones” designates different genera native to South Africa, belonging to the Aizoaceae family. It comprises 140 genera, including Dintheranthus, Lithops, Vanzijlia, Mitrophyllum, Cheridopsis, Fenestraria, Frithia and Mesembryanthemum.
Lithops (50 species) and Mesembryanthemum (350 species) are the most common. Unlike plants belonging to genus Mesembryanthemum, Lithops truly resemble living stones, because they are stemless.
Lithops grow in the dry deserts of South Africa, where they are regularly buried beneath the sand or surrounded by pebbles, which they resemble closely – hence their name. Their Latin name also underscores their special appearance (lithos = stone, opsis = appearance).
The plant is usually reduced to a single pair of opposite leaves, fused together in the shape of a cone with a slit down the middle from which the flower and the next pair of leaves emerge, at a right angle to the old pair of leaves. A new shoot is formed each year. Some species produce two pairs of leaves per shoot.
Because Lithops grow buried in the soil, the only part exposed to the sun is the flat top of their marbled or spotted leaves. Sunlight is absorbed through the apical surface of their truncated leaves all the way to the chlorophyllous tissue, passing through the transparent cells that entirely fill the leaves and store water. Cultivated specimens are usually grown with their tops farther out of the soil, however.
Fenestraria is a “window plant” that also grows buried in the soil. The tiny “windows” on the tips of its stick-like leaves let sunlight into the plant. If you look closely at one of these plants you will see what looks like a water droplet in the centre of each leaf – that is the tiny window.
In terms of numbers, Crassulaceae is the third-largest succulent plant family, with 35 genera and 1,500 species.
Plants in this family are found in the widest possible variety of habitats around the globe. All species have xerophilous leaves, which means that they are adapted to resist dry spells. The epidermal layer is covered with a thick cuticle.
The Botanical Garden collection includes some twenty species belonging to this family. The largest Crassulaceae genera, in addition to Crassula, are Kalanchoe (150 species) and Echeveria (150 species).
Euphorbiaceae is a large, cosmopolitan family comprising some 300 genera and 5,000 species distributed in all parts of the globe with the exception of the Arctic.
Genus Euphorbia alone includes 2,000 species, ranging from annual plants to tropical shrubs and trees.
The Montréal Botanical Garden collection contains 15 genera and 70 species.
Euphorbias are especially toxic plants.
In fact, when pruned even slightly they release a viscous white sap that is a skin irritant.
The scientific name of poinsettia, a well-known plant sold during the holiday season, is Euphorbia pulcherrima. These plants are the stars of the winter display in the Main Exhibition Greenhouse.
Liliaceae, the family that includes tulips and trilliums, also contains several genera of succulent plants.
Genus Aloe, well known for its medicinal properties, is also very popular with collectors.
This genus includes some 200 to 250 species native to Africa, Madagascar and Arabia. Most are shrub- or tree-like species with a rosette of leaves at the tips of their branches. Aloes have been grown for over 2,000 years in Egypt, where they were planted in cemeteries and other sites. The ancient Greeks were also familiar with their medicinal properties. A wide variety of products containing aloe extracts are commercially available today.
The Botanical Garden’s collection contains about 65 aloe species, some of them rare, including Aloe branddraaiensis, Aloe ciliaris and Aloe greenii, all native to South Africa.
These specimens are usually kept in the service greenhouses, which are not open to the public. Also belonging to the Liliaceae family is genus Haworthia, very popular with collectors, and which includes some 150 species, all native to South Africa. These are low-growing plants with dense rosettes of leaves. The flowers are borne in a long cluster, which is rarely ramified and fairly similar for all species.
Haworthias owe their popularity to their compact form and shade tolerance and the variety of leaf shapes and markings. The leaves of Haworthia truncata, for instance, are translucent, truncated and arranged in two rows facing each other. In the wild, the contractile roots pull the plant deeper into the ground, leaving only the translucent “window” on the leaves exposed to sunlight so that it can reach the inner tissues and photosynthesis can occur.
What is a caudiciform plant?
It is a plant with a swollen, perennial storage organ that allows it to withstand dry spells. This organ, the caudex, is located at the base of the plant and does not perform photosynthesis. It may be formed by the root system, the stem, or both.
Another characteristic feature is that the caudex is almost never green, articulated or covered in tubercles.
During the rainy season, temporary slender, fragile and often climbing photosynthetic structures appear on the caudex.
Caudiciform plants are found in various Angiosperm families, including Apocynaceae, Asclepiadaceae, Convolvulaceae, Dioscoreaceae, Euphorbiaceae and Vitaceae.
Based on an article by Denis Barabé and Marc Saint-Arnaud in Quatre-Temps magazine, Vol. 20, no.3