CBSE Class 12 Biology

## The flower as the reproductive unit A flower is the part of an angiosperm that carries out sexual reproduction. Its four whorls, from outside in, are the calyx (sepals), the corolla (petals), the androecium and the gynoecium. The last two are the reproductive whorls. The androecium is the set of **stamens**. Each stamen has a long, slender filament and a terminal anther, which is usually bilobed. The gynoecium is made of one or more **carpels** (together called the pistil), and each carpel has a stigma that receives pollen, a style that connects the stigma to the ovary, and an ovary that contains one or more ovules. When a gynoecium has several carpels they may be free (apocarpous, as in a lotus or a rose) or fused (syncarpous, as in a mustard flower). The male and female gametes are made by different structures and in different ways, and in both cases the cells involved pass through a sporophytic stage first. A **gametophyte** is the small haploid body that produces gametes, and in flowering plants it is reduced to a few cells living inside the flower. The pollen grain is the male gametophyte and the embryo sac is the female gametophyte. ## The anther and microsporogenesis A typical anther is bilobed, and each lobe has two chambers, so a cross-section is roughly four-sided with a microsporangium (pollen sac) in each corner. Each lobe is called a theca, so the anther is dithecous and tetrasporangiate. The four microsporangia are the sites where pollen grains develop. A…