Introduction

The Australian continent possesses a limited number of succulent species, and some of the most obvious ones are leaf succulents of the Aizoaceae and Portulacaceae families. A number of these produce large attractive flowers, making them worthy of a place in any succulent collection. However, few growers to date have considered the relatively large number of tuberous, cormous and bulbous plants that occur in Australia. Some of these have potential as interesting pot and garden subjects and the discerning succulent grower should not overlook or dismiss them. Often, the way in which we grow and display our plants is just as important as the plants themselves. Didge Rowe stated it aptly in his article on Mesembryanthemaceae in the November 2001 edition of Dinteranthus. "Growing specimen plants so that part of their tuberous root system shows above ground, can give very artistic and aesthetic results". And what better bonsai specimen is one that does not require daily watering?

There is enormous potential within the range of our unique and fascinating Australian flora, for the propagation of species that develop fat, swollen storage organs above and below ground. This article will look at the many forms of "edaphytic" succulents that occur in our Australian flora, and will examine their potential as specimens in the succulent collection.

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The Basics

John Pate and Kingsley Dixon in their work Tuberous, Cormous and Bulbous Plants, divide subterranean succulents into five basic types. This paper will not discuss the fifth type, as it involves the development of storage organs in seedlings. Also, it will only examine some of the types and sub-types briefly, as they contain species that are unlikely to be of interest to the serious collector of succulent plants.

1. Bulbs

A bulb is a modified perennating shoot that takes the form of a vertically compacted stem made up of overlapping swollen scales. These scales are receptacles for the storage of food and water, and are the equivalents of swollen basal leaves or specialised underground leaves, or a mixture of the two. The bulb can survive on its reserves during a long period of dormancy. Environmental factors that trigger dormancy may include regular seasonal patterns (eg. Winter) and/or long periods of dry weather or drought. During periods of vegetative growth, the meristem(s) or growth point(s) of the bulb elongate to produce above ground shoots. These typically comprise foliage, and a centralised scape bearing the reproductive structures (flowers) of the plant.

The reserves in the older scales gradually become exhausted and their withered remains persist on the outer surface of the bulb. Over progressive seasons or growth periods, a multi-layered, waterproof coating develops. New scales begin their life in the centre of the bulb and end their life on the outside of the bulb. Over progressive seasons or growth periods the bulb gradually increases in diameter. The humble garden onion is a good example of a bulbous plant.

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Examples of native bulbous plants include Crinum flaccidum (Murray lily), Proiphys cunninghamii (Brisbane lily), and Bulbine bulbosa (Native leek).

The individual bulbs of Crinum flaccidum can be massive and extend deep into the earth. The strap-like leaves and enormous white flowers appear during warm wet periods, sometimes lasting many weeks or even months. Most of the year the plant is nothing more than a huge white bulb that resides deep in its subterranean home.

Bulbs can sometimes extend above ground to form thick pseudostems. The bulky native species, Crinum pedunculatum, is a good example.

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2. Corms

A corm is a swollen, much compacted underground stem. The corm has a vertical orientation with a well-defined meristem at the top. Scar tissue along the outside of the corm marks points of attachment of previous vegetative shoots, inflorescences, and roots. A corm differs from a bulb in that it lacks scales, so the main storage tissue lies within the corm itself. The withered fibrous remains of leaves from previous growth periods can form a protective coating over some corms. During periods of vegetative growth, corms produce adventitious roots from the bottom and sides of previous or current-season nodal tissue.

Like a bulb, a corm produces vegetative aerial growth during the growing season, but, unlike a bulb, a flowering stem usually only develops some time later, when the plant has reached maturity and becomes reproductive. As they swell and elongate, corms often produce fresh nodal and internodal tissue on top of or on the side of old tissue. Over progressive seasons the corm may take on a gnarled, multi-jointed appearance. Older nodes on the corm may shut down altogether and the main growth and reproductive functions transfer to newer nodal tissue, such as that which developed during the previous season's growth.

Some cormous plants reproduce vegetatively by forming lateral offsets, which begin life as small buds attached to the outer surface of the parent corm. They can also reproduce by division of the meristem, before the addition of new storage tissue.

Taro is a good common example of a cormous plant. Examples of native cormous plants include Typhonium brownii (Stink lily), Curcuma australasica (Cape York lily), Alocasia brisbanensis (Cunjevoi), and Hypoxis occidentalis.

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3. Stem tubers

A stem tuber is any regular or clearly defined globoid, cylindrical or jointed swelling of underground fleshy stem tissue.

3.1 Modified underground shoots (jointed type).
In this type, the tubers comprise swollen underground shoots, modified to produce jointed, swollen storage tissue. The joints or internodes typically have a regular outline and the spacing is fairly even. The storage potential increases seasonally with the extension and thickening of the existing tubers or by the development of new tubers. Swollen and/or jointed shoots are a typical feature of some native grasses, of which Poa drummondiana is a fine example.

3.2 Perennial stem tubers arising on modified underground shoots (non-jointed type). Examples include: Stephania japonica (Tape vine), S. bancroftii, and S. sp. "tuberosa".

Stephania sp. "tuberosa" is an unnamed species from Cape York Peninsula that produces a massive above-ground caudex. This is a highly desirable species for the succulent enthusiast.

3.3 Paired stem tubers arising on modified underground shoots, eg. Drosera spp. (sundews), Dioscorea spp. (yams), and Tacca maculata.
This storage tissue takes the form of swollen, highly modified lateral shoots that arise separately on an otherwise non-swollen shoot system. In a typical growing season, a plant comprises a pair of stem tubers. One is a parent tuber, formed from the previous season's growth, and the other, a replacement tuber, forming and filling with new storage material. The pair occur vertically, often side by side, and originate from modified leaf axils. They emerge from opposite sides of a node located on the lowermost part of the stem. This side by side replacement strategy is common in species of Drosera (sundews) and Dioscorea (yams).

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The tubers of Drosera are typically globoid and in some species form within the epidermal sheaths left behind by previous seasons' tubers. In Drosera erythorhiza, a dropper develops in mid-season from a special node located just below a leaf node. It penetrates vertically 2-5 cm before swelling terminally to form a globose stem tuber. The meristem of the tuber initially points down, but the tuber displaces as it grows, until the meristem turns upright. At the end of the first season all plant parts die except for the tuber, and replacement tubers form in subsequent seasons, at progressively greater depth.

In Dioscorea hastifolia - a native yam - the tubers penetrate up to half a metre down before enlarging into a thick cylindrical structure. The stem tuber is often the only organ of these plants to survive between growing seasons.

3.4 Tubers that form as bulbils on stems above ground but later grow below ground.
The yam Dioscorea bulbifera from tropical areas, has developed an innovative reproductive strategy. It produces its marble-sized bulbils on aerial stems and, when mature, they drop to the ground to form new plants. This plant is very easy to grow in a pot or in the garden, but appreciates warm conditions, some moisture, and good light.

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4. Root tubers

Root tubers are swellings containing food and water reserves, which arise on existing "true" roots, or on specialised roots connected to the "true" roots. Root tubers may also occur on "adventitious" roots derived from stem tissue. In this case they may appear more like stem tubers than root tubers, and indeed their function is essentially the same. In a few species the distinction is not entirely clear.

4.1 Tubers representing regular or irregular swellings along part of a root system.
Many species of Australian plants possess swollen tubers on their root systems.

4.11 Tubers representing regular or irregular swellings along portions of a branched root system.
Examples include: Calothamnus tuberosus, Plectranthus mirus, Parakeelya primuliflora, Platysace deflexa (Bush potato), Cayratia clematidea (Native grape), Marsdenia australis (Bush banana), Brachychiton gregorii, and Cissus opaca (Bush grape).

The Western Australian shrub, Calothamnus tuberosus, is probably the most unusual, with a vast network of underground roots that resemble long strings of fat sausages. These roots spread out for many metres into the surrounding soil, seeking out moisture and nutrients to store.

Several species of native spur flower of the genus Plectranthus possess tuberous root systems. Plectranthus mirus is a small somewhat variable species, quite common in the wet tropics to the west and north of Cairns. I have observed this species in habitat on large granite rock platforms near Herberton. Plants at this location exhibited a surprisingly vast network of tuberous, yellow-coloured roots. In a few places, the tubers were lying across bare rock, completely exposed to the elements, but were elsewhere beneath a very thin layer of peaty humus and gravel.

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Several species of Platysace produce root tubers of various kinds. Platysace deflexa develops numerous small tubers irregularly along a suckering root system.

Cissus opaca is a locally indigenous "dry scrub" species, most commonly observed in brigalow communities as a small weakly climbing vine with shiny, digitately compound leaves. I suggest planting the long-lived tubers into shallow saucers or bonsai pots, with the tops of the large brown tubers left exposed. The grower can then train the leafy stems around a support and prune to shape. The flowers are small and insignificant, but a well-grown specimen displayed in this odd "bonsai" fashion can still turn a head or two. Cayratia clematidea is a similar species that could also be trialed in this manner. It has tan-grey globoid tubers, up to 5 cm in diameter, with a slightly warty surface texture.

Local aboriginal tribes gathered the tubers of Cayratia clematidea and Cissus opaca as "yams". They prepared the tubers of Cayratia clematidea by beating them on stones before roasting, but they apparently ate the tubers of Cissus opaca raw. However, a chemical examination of the tubers of C. opaca, conducted by Baker and Smith in 1906, showed them to have "comparatively little food value".

Several twining asclepiad species of the genus Marsdenia also produce tubers on their root systems. Marsdenia australis produces horizontal (taproot) and lateral root tubers, from which new plants (suckers) may emerge. The lateral tubers may be up to half a metre long and 25 mm thick and occur from 10 cm to 50 cm below the soil surface. The roots branch copiously to produce tubers in a number of horizontal planes. The vertical tubers may continue beyond 75 cm depth.

4.12 Tubers representing regular or irregular swellings along portions of an otherwise unbranched and unswollen taproot.
The Aboriginal bushfood plant, Platysace cirrosa (Kahno), is a small growing species that produces a series of perennial bead-like tubers along an otherwise unswollen taproot. The initial (shallowest) tubers are elliptic and only 1-2 cm thick, but the final (deepest) tuber is globose and up to 8 cm diameter. The stems of the plant are terete and mostly leafless and twine into surrounding vegetation to a height of about 30 cm. This remarkable little plant from SW Western Australia has potential for the succulent enthusiast, but sadly appears to be unavailable in cultivation at this time.

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4.2 Tubers formed by swelling of adventitious roots, eg. some terrestrial orchids, lilies, grass trees.
Adventitious roots are any roots of "nodal" character that have the appearance of being atypically or abnormally placed on a plant.

4.21 Tubers numerous, and added to seasonally as new adventitious roots form.
Typically the new tubers form above the older tubers. In mid-growing season the younger ones are distinguishable by their light colour and smooth surface and lack of fibrous feeding roots. The tubers range in shape from thick cylindrical to globose. Examples include: Burchardia spp. (Milkmaids), Arthropodium spp. (Chocolate lilies), Murdannia graminea (Grass lily), Thysanotus tuberosus (Fringed lily), Chamaescilla corymbosa (Blue squill), and Clematicissus angustissima.

Clematicissus angustissima has singular tubers that occur communally, attached in a cluster that radiates from the base of a narrow stem. The tubers are perennial, but added to seasonally by the formation of new tubers. The numerous oblanceolate tubers are 3 cm wide and 30 cm long. They cluster from the stem base at a depth of 10-15 cm. The plant is a deciduous woody perennial to 2 m tall, and grows amongst rubble and granite fissures in SW Western Australia.

4.22 Previous-season parent tuber replaced annually by a new-season tuber, eg. Pterostylis mutica (a terrestrial orchid).
These tubers are technically roots not shoots, although the storage and survival strategy are essentially similar to species bearing paired stem tubers (cf. native yam, Dioscorea hastifolia). Almost all root tuber plants in this category are shallow-rooted geophytes, and most are terrestrial orchids.

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4.3 Tubers mostly singular, occasionally bifurcated, persistent, ± perennial.
This type of tuber is the equivalent of a swollen taproot. It is not replaced annually or seasonally, but remains on the plant for its entire life.

4.31 Tubers mostly singular (but occasionally forked, or supplemented with a few swollen lateral protuberances), typically attached to the base of a much-reduced stem.
This kind of tuber forms what is essentially the main stem/root interface of the plant. Examples include: Grahamia australiana (Australian anacampseros), Parakeelya pleiopetala, Plectranthus glabriflorus, P. parviflorus (Native spur flower), Brachystelma glabriflorum, Sedopsis armitii, S. filsonii, S. sp. ("Bulimba"), Portulaca bicolor (pink and purple-flowered forms only), Portulaca australis (Native portulaca), and P. filifolia (Narrow-leaved pigweed).

Parakeelya pleiopetala is a small, tuberous plant of the Portulacaceae family. It will grow for many years in a shallow saucer pot in a well-drained mix. It likes plenty of sunshine, warm to hot conditions, and regular watering during the spring and summer months. From a dense tuft of narrow, succulent leaves, masses of lovely white flowers appear. Unfortunately, this plant can suffer from the dreaded SDS for no apparent reason.

Grahamia australiana is also in Portulacacea and requires similar conditions. In every sense, it is a true leaf, stem, and root succulent. It requires a long winter rest period, during which it may "die" right back to the tuber. Normally, the leaves and stems appear with the onset of rains in spring and remain on the plant throughout the long flowering period. Under ideal conditions, this flowering period may extend from spring through to autumn. However, Australia has an unreliable climate that commonly interrupts the growth cycle of its flora. As with many native plants, Grahamia has developed the adaptive strategy of an "opportunistic" growth habit. The Grahamia may remain dormant during long periods of drought, or it may become active and flower during an unseasonably wet winter. The species occurs sporadically from the tropics right through the sub-tropics and deep into the temperate zone, as far south as the Flinders Ranges. In tropical and sub-tropical areas, summer storms are the dominant rain source, but in southern climes, winter frontal rain becomes the norm. The only common climatic feature over this wide range of distribution is a general unreliability of rainfall.

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Grahamia grows best as a colony in a shallow saucer pot. Water it regularly during the spring and summer months, and it will also appreciate regular weak applications of a soluble fertiliser. Flowers are pinkish and open in the late evening. There appears to be a distinct form (or subspecies) in the wild, which has smaller leaves, shorter stems and pure white flowers. It is for pot culture only, as you would soon lose such a tiny plant in the garden!

Plectranthus glabriflorus is a small shade-loving plant from the Herberton district of north Queensland. Its diminutive size above ground is somewhat deceptive, because it produces enormous potato-like tubers just beneath the soil surface. The plant can "die" back to the storage tuber during the dry season. The main period of vegetative growth and flowering occurs during the summer "Wet". This is a somewhat rare species in the wild, but it is hardy and very accepting of pot and garden culture.

Plants in the Sedopsis/ Portulaca bicolor complex generally begin life with the development of a small globoid or elliptic root tuber that conjoins the stem base. In most species the tuber continues to swell to form an irregularly globoid-cylindrical mass, or, in the case of Sedopsis spp., it typically elongates to form a long, somewhat contorted, cylindrical structure. In all species, this primary tuber may show some bifurcation and additional recruitment of lateral tubers. Where plants have developed on solid rock pavement, the tubers commonly lie flush against the rock surface, covered only by a very thin layer of peaty gravel.

Sedopsis sp. ("Bulimba") is an unnamed species from the Atherton Tablelands of northern Queensland. It is a tiny plant that produces bright purple flowers up to the size of a ten cent coin. It can be difficult in cultivation, as it is very fussy about its growth requirements. It grows naturally in sun or semi-shade on sloping granite rock platforms, often in small deposits of peaty gravel. It therefore appreciates a very shallow pot, excellent drainage, regular watering in summer, high light and good air circulation.

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Portulaca australis and P. filifolia require similar conditions to the Grahamia and Parakeelya, but are generally short-lived, so the grower must retain seed for future cultivation. An alternative is to do a group planting in the garden and allow them to seed naturally. Flowers of P. filifolia are yellow, small and star-like, and the plant is fairly weedy and nondescript. P. australis, on the other hand, is slightly more compact and has broader flowers, (up to 2 cm diameter in some forms, and with colours ranging from bright yellow to apricot). Such forms are worthy of cultivation.

Both species possess an oblong taproot tuber, although some specimens may show bifurcation, or recruitment of supplementary tubers along portions of lateral roots. Furthermore, some individual specimens of P. filifolia do not possess any tubers, and such plants have apparently developed under conditions that did not require the formation of storage units. It seems likely that, in this species, one or a combination of environmental stresses are responsible for triggering the development of tubers, particularly during the early part of the plant's life. Environmental stresses may include drought, seasonal changes, low nutrient levels, and extreme heat or cold.

The red-stemmed "inland" form of the common pigweed, P. oleracea (Munyeroo), will also develop a stress-induced tuber if environmental factors interrupt growth during the formative period.

All native Portulaca species possess succulent leaves and stems, and they share that typical succulent habit of flowering during the hottest part of the day, in the hottest months of the year. In Australia, the genus consists of species that are true annuals, annuals with biennial or perennial tubers, and tuber-bearing perennials.

4.32 Tubers mostly singular (but occasionally forked at base, or supplemented with a few swollen lateral protuberances), typically attached to the base of an unswollen underground taproot.
Examples include Pentapeltis silvatica, P. peltigera, Cayratia trifolia, Ipomoea gracilis and I. longiflora. This category is essentially similar to the previous one. It differs only in so far as the tuber adjoins the base of the unswollen part of a taproot and not the base of a much-reduced stem. In this case it is the unswollen part of the taproot that adjoins the base of the stem.

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4.33 Tuber a greatly thickened taproot, (sometimes bifurcated, or with a few swollen lateral roots), not attached to a much-reduced stem or unswollen taproot, but bearing a "crown" of typical leaves and stems.
This type is similar to 4.32 and 4.31, but in this case the tuber does not adjoin the base of a much-reduced stem nor the base of an unswollen taproot. The tuber is a generalised enlargement of the entire taproot. The caudex of the tuber rests at or just below ground level. The tuber is generally carrot-shaped and possesses a "crown" from which typical leaves and stems emerge.

A common example of this type of storage unit is the garden carrot. Examples of native species include Trachymene hemicarpa and T. incisa (Native parsnips), Geranium solanderi (Native geranium), and several other Geranium spp. Some native daisies, eg. Glossocardia bidens, also have carrot-like tubers.

Trachymene incisa occurs in places like the Granite Belt of southern Queensland. Here it grows in fissures on exposed granite rock platforms, often at high altitude. It withstands icy gales and heavy frosts through winter and baking heat in summer. The flowers are most attractive, being large white umbels produced on the ends of tall, rigid peduncles. This is another plant for the succulent enthusiast to try in a small pot with a few centimetres of the caudex exposed. There are other Trachymene species that also possess this swollen taproot.

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Acknowledgments

Special thanks must go to Paul Forster and John Story, for answering the many difficult questions I put to them during the writing of this paper. Paul Forster also provided copies of relevant articles from the BRI library.

References and further reading

Baker, R.T. and Smith, H.G., "Vitis opaca, F.v.M., and a Chemical Investigation of its Enlarged Rootstock (Tuber)." Read before the Royal Society of New South Wales, August 1, 1906. In Journal and Proceedings of the Royal Society of New South Wales 40:52-60, 1906.

Forster, P.I., Notes on Anacampseros australiana J.M.Black (Portulacaceae). Anacampseros 3:48-55, 1987.

Forster, P.I., Brachystelma glabriflorum (F.Muell.) Schltr. (Asclepiadaceae). Asklepios 49: 77-84, 1990.

Forster, P.I., Australian Native Succulents. Cactus & Succulent Journal (New South Wales) 23:12-18, 2001.

Hershkovitz, M.A., Parakeelya: a new genus segregated from Calandrinia (Portulacaceae). Phytologia 84:98-106, 1998.

Jones, D.L., Native Orchids of Australia, Reed, Australia, 1993.

Menkins, I., Notes on the Cultivation of Native Plectranthus. Dinteranthus 15 (3):80-81, 2001.

Menkins, I., A Brief Look at Plectranthus in Australia. Dinteranthus 15 (3):81-83, 2001.

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Nichols, K.M., Browne, J.H., and Parsons, R.F., Ecology of two asclepiad lianes in semi-arid Victoria. Proceedings of the Royal Society of Victoria 103 (2):93-112, 1991.

Pate, J.S. & Dixon, K.W., Tuberous, Cormous and Bulbous Plants - Biology of an Adaptive Strategy in Western Australia. University of Western Australia Press, Nedlands, 1982.

Pate, J.S. & Dixon, K.W., Tuberous, Cormous and Bulbous Plants. Australian Plants, 17 (133):30-39, 1992.

Rowe, D., Mesembryanthemaceae - a fascinating plant family: An introduction to the plants and their culture. Dinteranthus, 15 (3):68-73, 2001.

Stanley, T.D. & Ross, E.M., Flora of south-eastern Queensland Volumes I, II, & III, Queensland Department of Primary Industries, Brisbane, 1983, 1986, & 1989.