Peltate: On a stalk attached to the lower surface of the leaf rather than to the base or margin of the leaf. Sagittate: With a downwardly pointed lobe on each side of the base. Pinnatifid: Pinnately lobed half the distance or more to the midrib, but not all the way to the midrib. Spadix: A spike with small flowers crowded on a fleshy axis, characteristic of the Araceae family (the "jack" in the pulpit [spathe]). Spathe: A large bract beneath and enclosing the inflorescence (flowering part of the plant). Petiole: Leaf stem.
Generally (not always) Colocasia has a partially peltate leaf blade, while Alocasia leaf blades can be any shape imaginable from entirely peltate to sagittate, to deeply pinnatifid.
Generally (not always)Colocasia has a small sterile region at the tip of the spadix, while Alocasia has a longer sterile region at the tip of the spadix.
Colocasia produce inflorescences in a fan running away from the petiole from which they are being produced (i.e. the back of the spathe faces the petiole from which it is produced), and, generally, produces several inflorescences per leaf. Alocasia produces inflorescences running parallel to the leaf and, generally, only one or two per leaf.
Alocasia produces rather large, odorless berries carried erect, that ripen orange-red, each containing one to three (rarely more) large, round seeds. Colocasia produces small, yellow-brown, fruits on a nodding stalk that have a fruit scent and contain many, small, ellipsoid seeds.
Alocasia come in all sizes. Alocasia robusta, is a giant, endemic to Borneo from forest clearings to stream sides. It is said to have the largest undivided leaves in the world, which can reach up to thirteen feet (4 m) in length. From New Guinea, Alocasia portei can reach ten to fifteen feet (3.04 - 4.57 m) in height, forming a trunk that is almost palm like. It is said to be similar to A. macrorrhiza, but with deeply lobed leaves held upright.
At the other end of the spectrum, the leaves of Alocasia reginae mature at about ten to twelve inches (25 - 30 cm). The intense silver of these leaves is fantastic. Picture this growing out of a swath of one of the purple leaf sweet potatoes! (Ipomoea batatas)
Alocasia macrorrhiza 'Variegata' is going to be my first Alocasia. Just look at that leaf! The leaves were easily over three feet long (.91 m).
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