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Friday, 28 Apr 2017

The Family Aroids or Araceae

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The Araceae, or aroids, is a family of herbaceous monocotyledons with 104 genera and about 3700 species if the Lemnaceae (duckweeds) is not regarded as a generic synonym, or 108 genera and about 3750 species if the Lemnaceae are included. The family is predominantly tropical in distribution, with 90% of genera and c. 95% of species restricted to the tropics. Although the greatest number of species originate in South America (including the two largest genera, Anthurium and Philodendron with over 1500 species between them, the tropics of South East Asia are also very rich, with the large and horticulturally important genera Alocasia and Amorphophallus

 

The Araceae contains several well-known cultivated foliage and flowering plants. e.g., Philodendron, Monstera, Spathiphyllum and Anthurium. A number of important food crops belong to the Araceae, e.g., taro (Colocasia esculenta), tannia or cocoyam (Xanthosoma sagittifolium), elephant yam (Amorphophallus paeoniifolius), konjac (A. konjac) and giant yam (Cyrtosperma merkusii).

 

Members of the family are highly diverse in life forms, leaf morphology, and inflorescence characteristics. Life forms range from submerged or free-floating aquatics to terrestrial (sometimes tuberous), and to epiphytic or hemiepiphytic plants or climbers. Leaves range from simple and entire to compound and higly divided, and may be basal or produced from an aerial stem. The family Araceae is defined by bearing small flowers on a fleshy axis (spadix) subtended by a modified leaf(spathe). There is much variation on this theme.

 

In some genera the spathe is very conspicuous and brilliantly coloured (e.g., many Anthurium species) while in others the spathe is small and leaf-like (e.g., many Pothos species). In the North American genus Orontium the spathe is so reduced that it appears to be absent altogether and in Gymnostachys, a peculiar genus restricted to eastern Australia, debate continues as to whether a spathe is in fact present or, indeed, if Gymnostachys might be better removed altogether from the aroids. The behaviour of the spathe varies from genus to genus. In some (e.g., Cryptocoryne) the spathe completely encloses the spadix, while in others the spathe reflexes to leave the spathe clearly visible (e.g., most Anthurium, Spathiphyllum). In some genera the spathe is shed as soon as the inflorescence reaches anthesis, either falling completely (e.g., Rhaphidophora) or partially (e.g., Schismatoglottis). The spathe ranges in size and shape from 5mm long and simple in Homalomena humilis to the fluted and pleated vase 1m wide and 1.5m tall found in Amorphophallus titanum.

 

The sex of the individual flowers and their arrangement on the spadix are among the characters used to define taxonomic groups. Depending on the genus the spadix may bear either unisexual or bisexual flowers. If bearing bisexual flowers there are uniformly arranged over the spadix. Almost without exception bisexual flowers are subtended by reduced tepals termed a perigon. If unisexual, the flowers are usually arranged with the females at the base of the spadix occasionally terminated by a sterile appendix. In the genus Arisaema individual inflorescences are usually either male or female. The sex of the inflorescence in Arisaema is governed by the age of the plant, its health, and the type of conditions in which it is growing.

 

Young plants or mature plants in poor condition or growing in a less than ideal habitat will produce male inflorescences. The ability to alter the sex of the inflorescence in this way is termed paradioecy. Unisexual flowers are almost without exception naked. i.e., lacking a perigon.

 

The most recent technical account for the genera is The Genera of Araceae (Mayo, Bogner & Boyce, 1997) while a species checklist for the family, World checklist and bibliography of Araceae (and Acoraceae) by Govaerts, Frodin etal., appeared in 2002. Aside from floristic accounts and taxonomic treatments the best non-technical account is that of Bown (2000). The classic work on the genus Arum, Lords and Ladies (Prime, 1960), is essential reading for anyone wanting to understand the pollination strategy employed by many monoecious aroid taxa.

 

All Araceae studied to date display insect pollination. Many, notably Amorphophallus have evolved to be pollinated by insects attracted to dung or carrion (saproentomophily). Many tropical species have inflorescences where pollination has evolved in conjunction with bees, wasps and beetles. In species of Philodendron investigated to date large dynastid scarab beetles are attracted to the inflorescences and appear to be the main pollinators (Gottsberger & Amaral, 1984). Many aroids attract pollinators by odour. Inflorescence odours include dung, carrion, rotting fruit, old socks, semen, bad breath, beer, spearmint, cheap sweets and cinnamon.

Several general have inflorescences that heat up considerably during anthesis, often by as much 20°C above the ambient temperature and often producing at the same time a strong, foul odour. Some genera also offer potential pollinators food in the form of fat bodies (Dieffenbachia), sugar solutions (many Arum species) or oil droplets (Amorphophallus).

Horticultural and scientific interest in the family is well supported by the The International Aroid Society and anyone with an interest in aroids in recommended to join with membership bringing regular newsletters, the journal Aroideana and access to an electronic newsletter forum, aroid-l.

 

References
Bown, D. (2000). Aroids. Plants of the Arum family, 2nd ed.. Timber Press. Portland.
Boyce, P.C. (1993). The genus Arum, Kew Magazine Monagraph. HMSO.
Gottsberger, G. & Amaral, A. (1984). Pollination strategies in Brazillian Philodendron species.
Bericht der Deutschen Botanischen Gesellschaft 97:391-410
Prime, C.T. (1960). Lords and Ladies. Collins. London.