Melastome flowers have a recognisable look. Once you know what to look for, you can often recognise a member of the family from a single flower, even if you cannot name the species. This guide walks through the main parts.
The overall plan
A typical melastome flower is radially symmetrical, with four or five petals, though some groups have three or six. The petals surround a ring of stamens, and at the centre sits a single style. Below the petals, you will usually find a cup- or urn-shaped structure.
The hypanthium
The cup-like structure below the petals is called the hypanthium, or floral tube. It is formed from the fused bases of the sepals, petals and stamens. In melastomes it often surrounds the ovary, and it often carries hairs, bristles or scales, which is one reason many species feel rough to the touch. In many melastomes the ovary is inferior or partly sunk into this tube, a trait shared with several related families.
The stamens
The stamens are the showiest and most diagnostic part of the flower.
- Number: commonly twice the number of petals, so eight or ten in a four- or five-petalled flower.
- Poricidal anthers: the anthers do not split open along their length. They release pollen through small pores at the tip, so pollen comes out only when the flower is shaken, a feature linked to buzz pollination.
- Connective appendages: the connective, the tissue that joins the pollen sacs, is often extended into curved hooks, spurs or lobes. These give melastome stamens their characteristic bent, ornamented look.
- Dimorphic stamens: in many species, the flower has two sets of stamens that differ in size, colour and role. One set provides pollen as food for visiting bees. The other set is positioned to deposit pollen on the bee's body where it can reach another flower's stigma. This division of labour is called heteranthery.
Why two kinds of stamens?
Bees collect pollen to feed their larvae, so a flower that gives away all its pollen would leave little for pollination. By producing a "feeding" set and a "pollinating" set, a plant can reward visitors while still placing some pollen where it will actually be transferred. In some species the feeding stamens are brightly coloured, perhaps drawing bees to the right spot.
The ovary and fruit
After pollination, the ovary develops into either a capsule, a dry fruit that splits to release fine seeds, or a berry, a fleshy fruit eaten by animals. This difference is one of the main characters used to divide the family into groups. See seed dispersal in melastomes.
Variations
Not every melastome follows the pattern. Axinaea, for example, has swollen, food-rich stamen appendages that birds squeeze (Axinaea). Sonerila has three petals (Sonerila). Some species lose or reduce one stamen set entirely.
A quick field checklist
- Four or five petals around a ring of curved stamens.
- Stamens with appendages or hooks.
- Anthers with a single pore at the tip.
- A cup-shaped base that may be hairy.
- Leaves with strong longitudinal veins (identifying a melastome).
Takeaway
Melastome flowers combine a few simple elements into a surprisingly complex and diverse design. Their anatomy tells a story about pollination, and about how the family has diversified.