Inflorescence and the flower

A flower is really a modified shoot: at flowering time the growing tip of the shoot turns into a floral meristem and makes a flower. How the flowers are arranged on their stalk is the inflorescence. In racemose inflorescence the main axis keeps growing, and flowers form on its sides, the oldest at the bottom (acropetal order). In cymose inflorescence the main axis ends in a flower, so growth stops, and the flowers open from the top downwards (basipetal order).

A typical flower has four rings (whorls) on a swollen stalk end, the thalamus: the calyx (sepals) and corolla (petals), which are the accessory whorls, and the androecium (stamens, male) and gynoecium (carpels, female), the reproductive whorls. When sepals and petals look alike, as in lily, they are together called the perianth. A bisexual flower has both stamens and carpels; a unisexual flower has only one.

Symmetry of a flower:

• Actinomorphic: can be cut into two equal halves through the centre in any direction (mustard, datura, chilli).
• Zygomorphic: only one cut gives equal halves (pea, gulmohur, bean, Cassia).
• Asymmetric: no cut gives equal halves (canna).

Flowers may have parts in threes, fours or fives (trimerous, tetramerous, pentamerous). A flower with a small leaf (bract) at its base is bracteate; without one, ebracteate.

The position of the ovary compared with the other parts:

• Hypogynous: the ovary sits at the top, with the other parts below it; the ovary is superior (mustard, china rose, brinjal).
• Perigynous: the other parts arise from the rim of a cup around the ovary, at about the same level; the ovary is half inferior (plum, rose, peach).
• Epigynous: the thalamus wraps fully around the ovary and the other parts arise above it; the ovary is inferior (guava, cucumber, the ray florets of sunflower).