Many plants around the world form relationships with ants to the benefit of one or both of the species.

Etymology of the Genera

Myrmecodia:   from the Greek Myrmedodes meaning “full of ants”.   About 25 species are recognized

                        now. . More than 100 had been described but the 1993 revision of the genus did a lot of

                        lumping together of species.

Hydnophytum:  from the Greek hydnon for “truffle” and the Greek phyton for “plant” - for the

truffle-like caudex of small plants.  About 50 species of the 110 named will survive the pending

revision of the genus.

Myrmephytum:  from the Greek myrmex for “ant” and the Greek phyton for “plant”.   8 species

Anthorrhiza:     9 species with only one of those entering cultivation in early 2017 as seeds..

Squamellaria:  from the Latin squamella meaning “little scale” – for the 4 small, fringed plates inside

                        the petals of some species.  12 species are native to Fiji, Vanuatu and the Solomon Islands.

DISTRIBUTION

These genera grow in tropical Southeast Asia from the Andaman Islands on the west to Fiji on the east. The northern part of their range includes the Philippines and extends into Thailand, Cambodia and Vietnam. The York Peninsula of Australia is the southern edge of their range.  The highest diversity of Rubiaceous ant-plants is on the island of New Guinea.

Typical habitat is tropical rainforest, mangrove swamps, orchards and other forested habitat.

EPIPHYTES

With the exception of a few species that grow in rocky soil above 2400 meters all the Rubiaceous ant- plants grow as epiphytes.  Epiphytes are plants that live on the surface of other plants – usually trees. One advantage to the epiphyte is better access to sunlight in the canopy of the forest.

The roots of epiphytes have the job of attaching the plants to the trees. This is important because falling off the tree is death for an epiphyte.  They cannot grow in regular soil because the roots rot. The epiphyte’s roots tend to grow into nooks and crannies in the bark to secure the plant and sometimes roots will grow all the way around the trunk, encircling it. 

The roots still have the job of absorbing water and minerals for the epiphyte.  In habitat epiphytes tend to grow on certain species of trees that have “spongy bark” that holds water between rainfalls.  For Rubiaceous ant plants some of these preferred trees include: Melaleucas, Casuarina papuana, Banksia dentate, Terminalia catappa, Calophyllum inophyllum and Mangroves.      

In cultivation most ant plants are being grown in pots using a typical epiphyte mix like the ones used for growing epiphytic orchids in cultivation.  These mixes are open and loose and contain materials like fir bark pieces, long fiber sphagnum, perlite, charcoal pieces and coconut husk chunks.

HYDNOPHYTUM VS. MYRMECODIA

Fig. 1  A plant of Hydnophytum moseleyanum in a 5 inch pot                                 Fig. 2  A plant of Myrmecodia tuberosa in a 4 inch pot

                               Hydnophytum                                                              Myrmecodia

                                      - multiple normal size stems from top                                    - one (few) thick stems

                                      - spines on a few species                                                        - spines (modified roots) on most species

                                      - sessile or stalked flowers                                                     - flowers sunk in a cavity (alveoli)

                                      - 2 (a few species 4) seeds per fruit                                       - 4 to 8 seeds per fruit

                                      - ant entry holes scattered on caudex                                   - ant entry holes often in a horseshoe pattern

                                                                                                                                           - some species have shield-shaped bases called

                                                                                                                                              clypeoli on the stem at the bottom of the petiole.

FLOWERS, SEEDS AND FRUITS

For both Hydnophytum and Myrmecodia the flowers are small (3 to 16mm), usually white, and have the petals in a tube for most of their length. In Hydnophytum the petal tips will spread wide for the one day the flower is open.  In Myrmecodia the petal tips just barley part to form a very small entry hole.

The flowers are self-fertile, something that is common in plants that house ants since the ants will chase off pollinators too in defense of their home plant.

Fruits of both genera are small, sweet, fleshy drupes that are usually orange or red (white in Myrmecodia beccarii).  The fruits are eaten by birds.

The inner layer of the fruit (endocarp) is actually a hard covering on the seed, so what looks like the seed in a fruit is actually called a pyrene.  The pyrene has a sticky strand of flesh attached to it that aids in dispersal by sticking to bark or a branch if the bird scrapes it off its beak because it got stuck there as it ate the fruit.  Iridomyrmex cordatus ants also disperse the seeds by incorporating them into the carton (a mache’ of vegetable fragment) runways they construct.  The seeds germinate and flourish on the carton.

THE ASSOCIATION BETWEEN THE ANTS AND THE PLANTS

Symbiosis is two different species living together for the benefit of one or both.  There are many kinds of symbiosis. In this case the relationship is what is called mutualism – the type of symbiosis in which both species benefit in the relationship.

The benefits to the ants:

• A dry, dark place to live and breed in the rainforest.  Ants require darkness to raise their larvae and pupa.
• The ants eat a sweet nectar produced by Hydnophytum plants at the base of developing fruits after the petals have fallen off. Squamellarias also make special necteries to feed the ants.

The benefits to the plants:

• The plants absorb nutrients needed for growth from decaying waste products that the ants leave in some chambers of the caudex.  The wastes the ants leave include insects, insect fragments and ant feces.
• The ants defend the plant from other insects who come to eat the plant they live in.  There is considerable variation from plant to plant in how aggressive the ants are in doing this – based on the species of ant living in the caudex and other factors.

THE ANT COLONY

The caudex of some Hydnophytum and Myrmecodia plants get quite large.  I have had first-hand descriptions of “large pillow-size caudexes” from a friend who saw ant-plants in habitat.  In cultivation I have seen a 28 inch diameter caudex on a Hydnophytum formicarum and a football size caudex on a Myrmecodia tuberosa.   With such large caudexes an ant colony may inhabit a single plant.  More commonly a single ant colony will occupy a number of clustered ant-plants – even ant-plants of different species or families.

A small percentage of Rubiaceous ant-plants in habitat contain no ants.  Experiments suggest that these plants will grow more slowly and be less vigorous.¹  In cultivation ant-plants require no ant presence and will grow just fine if fertilized to provide the nutrients the ants would have provided.  Ants are opportunistic and will move into cultivated plants in homes and greenhouses.

The ants that occupy the plants in habitat are all capable of living in other situations - rotting logs, ground cavities etc.  No species of ant requires Rubiaceous ant-plants for their home and survival.

                               ¹ Huxley, C.R., 1978.  The ant-plants Myrmecodia and Hydnophytum and the relationships between   their morphology and occupants,  

                                                                      physiology and ecology.  New Phytologist  80:231-268

THE DOMATIUM: (a part of a plant that has been modified to provide protection for insects)

The caudex the ants live in is produced by the plant whether ants are present or not.  It is formed by a section of the baby plant in the germinating seed called the hypocotyl and it is formed and visible in the first few days after germination of the seed.  By the time the baby plant’s caudex is 5 mm wide at about two months old the caudex has already formed (or is forming) its first ant chamber and first ant entry hole!  The plant is still photosynthesizing with its cotyledons and has not made its first true leaves yet!  This suggests just how important the ant association is to the plant’s survival and success.

The first ant entry hole into the caudex is always on the underside of the caudex and the first internal chamber is usually an upside-down J shape. This first chamber and subsequent chambers are produced whether or not ants are present.  The ants play no part in their formation but may carry out the dried up tissue that died to form the chambers and tunnels.

TWO KINDS OF CHAMBERS IN THE DOMATIUM

Especially in Myrmecodias, two different kinds of chambers are formed in the caudex.  One kind has smooth walls and the other kind has growths on the walls that are called “warts”.  (See figures 3 to 5)

The ants raise their larvae and pupa in the smooth-walled chambers and leave waste material in the warted chambers where the material decays and releases nutrients like nitrogen, phosphorus and potassium.  Experiments with radioactive tracers have shown that these nutrients are absorbed by the warts and dispersed throughout the plant.¹

                                                                  ¹ Huxley, C.R., 1978.  The ant-plants Myrmecodia and Hydnophytum and the relationships between   their morphology

                                                                                                           and occupants, physiology and ecology.  New Phytologist  80:231-268

Fig. 3  Cross-section of a 2.5 inch diameter caudex of Myrmecodia tuberosa.  Note that most of the chambers have smooth walls.

Fig. 4  A different cross-section of the same caudex as above. An obvious warted chamber is in the center of the caudex.  The chambers to the top and right are recently formed and still have the dried-out parenchyma tissue in them. This eventually totally disintegrates or is removed by ant inhabitants.

Fig. 5  Photo of the wall of a warted chamber taken with magnification using a dissecting microscope.

HOW THE TUNNELS AND CHAMBERS FORM IN THE DOMATIUM

The caudex is originally formed as a solid structure composed of parenchyma type cells.  The cavity and tunnels form when meristematic tissue (tissue capable of rapid cell division) proliferates as sheets of cells that form pockets and tubes within the caudex parenchyma tissue. These sheets of cells then become waxy and block the flow of water and nutrients to the parenchyma cells within the pockets and tubes.  Those parenchyma cells die and dry up to form the tunnels and chambers. Some of these tunnels connect to the outside skin of the caudex and become ant entry holes on the caudex when the cells dry up and die.

Each year as the caudex grows bigger, new tunnels and chambers are formed with some connecting to preexisting chambers and some also making more new ant entry holes. 

In general the internal structure of the caudex is simpler in Hydnophytums and by my observations slower to develop after the initial few sets of chambers are in place.  Myrmecodias develop more complicated tunnel and chamber systems.

THE ANT INHABITANTS

Ants live in the caudex of most species.

• In habitat, specific ant species live in the caudex of Rubiaceous ant-plants:
  • Usually one of three species of Philidris (Iridomyrmex) ants.
    • The most common ant inhabitant is Philidris (Iridomyrmex) cordatus in open lowland habitat over most of the ant-plant range. This species eats insects but also visits the base of the ant-plant fruits to eat the nectar there.
    • 2 types of Philidris (Iridomyrmex) scrutator in denser rain forest and at higher altitude, above 2000 meters.
    • Philidris (Iridomyrmex) nagasau in Fiji.
  • In Australia the widespread tramp ant Pheidole megacephala is replacing Philidris (Iridomyrmex) ants.  And on Borneo Crematogaster difformis ants are fairly common.
  • Near roads and areas of human influence opportunistic ants are more likely.  These can be native or non-native species of ants.

• In cultivation, opportunistic species of ants will move into the caudex including Crazy Ants, Paratrechina longicornis.  These are small ants with long antennae that display erratic and rapid movements.

OTHER INHABITANTS

• Fungi, nematodes, fly larvae, beetles, roaches, and other invertebrates are found regularly in the chambers and tunnels of Rubiaceous ant-plants.

• A particularly complex and interesting symbiotic relationship occurs in Myrmecodia beccarii plants in Australia.  The larvae of the endangered Lycaenid butterfly Hypochrysops apollo feed on leaves and tubers of Myrmecodia beccarii.  The interesting twist is that the butterfly larvae are tended by the Philidris (Iridomyrmex) cordatus ants that are also living in the caudex instead of being destroyed or run off by the ants.

• When the ant entry holes in the caudex are larger than the usual 3 to 8 mm diameter, other larger inhabitants are more likely to be living in the caudex.  Some Hydnophytums in particular have entry holes up to 3 cm in diameter
  • Skinks nest in Hydnophytums near Port Moresby, Papua New Guinea.
  • Lizards can occur and frogs are fairly common, especially in some montane species of Hydnophytums with large entry holes.  In some of these Hydnophytums rainwater fills some of the caudex chambers and frog larvae live in the flooded chambers.

THE BIGGER PICTURE

The relationship that the Rubiaceous ant-plants share with the ants is duplicated by other genera of plants living in the same trees as the Rubiaceous ant-plants.  These include Lecanopteris ant-ferns and Dischidias.   In fact, these genera also share the ant colonies!  Even with as large as some Rubiaceous ant-plants are, it may take tens or hundreds of plants in any one tree or group of trees to house a single ant colony.  And those tens or hundreds of plants that a single colony of ants may occupy can include many plants of all three kinds – Rubiaceous ant-plants, Lecanopteris ant-ferns and Dischidias.

It is well known that carnivorous plants grow in nutrient poor soils where many other plants cannot grow because the carnivorous plants can supplement their nutritional needs with the insects they capture.  Rubiaceous ant-plant/Lecanopteris/Dischidia communities can be found from sea level Mangrove swamps right up through the various forest types and up to the tree line on the highest mountains.  But they are generally considered to be their richest and at their best in nutritionally poor forests - places like forested bogs and forests on limestone soils.  In these cases the high water table and high pH of the soil makes it difficult for plants to absorb nutrients.  This results in less nurtition and nutrients moving through the entire ecosystem because of less leaves, fewer insectc less detritis etc.  The extra nutrients the ant waste provides gives the ant-plant community an advantage similar to that of the carnivorous plants.

LECANOPTERIS ANT-FERNS

Lecanopteris is a genus of 14 species of epiphytic ferns growing in tropical and tropical montane habitats on the islands of Southeast Asia. .

Their claim to fame is that all but one of the species has enlarged rhizomes with internal chambers that are occupied by ants in habitat.  The one other species (Lecanopteris mirabilis) has a woody arched rhizome up to 8 inches across that ants live under, in the space between the arched rhizome and the bark of the tree the plant is growing on.  As with the Rubiaceous ant-plants the ants and plants share a mutualistic advantage.  The ants get a dry, dark place to live; the plants get nutrients from decaying ant wastes in some of the chambers and some protection from herbivores. 

Lecanopteris rhizome chambers are simpler than those in the Rubiaceous ant-plants. Nutrients still get absorbed by the plant but there are no warts in the chambers to facilitate absorption.

“Although it is rare to find Lecanopteris plants with ant-free domatia, the relationship between Lecanopteris and the ants is facultative rather than obligatory. Each partner can survive without the other, though possibly survival and vigor of the plants may be affected in the absence of ants. Although the ants are not dependent on the rhizomes for nesting, they will take advantage of them whenever they are available”^.2

In studies of 7 Lecanopteris species, over 30 species of ants were recorded as visiting the plants including the same ones found in the Rubiaceous ant-plants.²

             ² British Natural History Museum web page titled “Ant-fern Association”:   http://www.nhm.ac.uk/nature-online/species-of-the-day/collections 

                             /our-collections/lecanopteris-spinosa/ant-fern/index.html   (no longer online as of 3/1/2016)

Fig. 12  This is a photograph of a cross-section cut through the rhizomes of a mature plant of Lecanopteris curtisii showing the chambers that the ants inhabit.  Notice the dark lining of the chambers. Old sections of rhizome also turn black.  Ants require darkness to breed and raise their young.

DISCHIDIAS

• Dischidia is a genus of plants in the Milkweed family, Asclepiadaceae (now Apocynaceae).
• The genus is closely related to Hoyas. 
• There are about 80 species of Dischidias which all grow as epiphytes and are native to tropical areas of China, India and most of the same areas of Southeast Asia as Rubiaceous ant-plants.
• Many Dischidias grow in arboreal ant nests and some have developed a symbiotic relationship where the plant makes modified leaves to provide housing for the ants. The ants in return protect the plants and leave waste to decay which provide the plants with nutrients.
•  

Dischidias can be divided into 3 groups based on their degree of involvement with ants:

   1. At least three species of vining Dischidias develop what are called bullate leaves which are hollow and have a single ant entry hole.  Ants live inside of these leaves.  As the leaves fill up with the ant’s wastes the ants move to other living spaces and the plant grows a root into the leaf that absorbs nutrients from the waste.

 These Dischidias also produce normal leaves in addition to the bullate leaves.

Fig. 13  Dischidia major showing the small normal leaves that are about the size of a quarter and the enlarged, hollow leaves that the ants live in.

2. A number of other Dischidia species make imbricate leaves which hold tightly to the growing surface (tree branch, tree trunk etc.). The underside of the leaf has a space where the ants live and leave their wastes.  Plants with this type of growth habit are sometimes called Shingle Plants.

Fig. 14  Dischidia species from Camiguin Island in the Philippines.  The leaf pairs are about 3 inches across.

 3. The third group of Dischidias that are involved with ants do not house the ants.  The plants are found growing on arboreal ant nests because the ants collect their seeds and bring them back to their nest where they sprout and grow^.3  The hypothesis is that the seeds smell like some food that the ants favor so they pick them up and bring them back to the nest.   Dischidia astephana is one of this kind of Dischidia. 

                ³  New Scientist, June 5, 1986, page 29.

                                                          Fig. 15  Dischidia astephana