Define angiosperms.

Hint: Angiospermae is also known as flowering plants, the most varied group of land plants. These are seed-producing plants. They are differentiated from gymnosperms, by features like flowers, producing fruits containing seeds, and the presence of endosperm within the seeds.

Complete answer: Plants that produce flowers are called angiosperms. In angiosperms, the flowering organs or the reproductive organs are the most distinguishing feature from other seed plants. These have a more species-specific breeding system and hence, they evolve with greater species diversity and help in the speciation process. Two pairs of pollen sacs are present in stamen and are much lighter than gymnosperms. Stamens have been modified as an adaptive feature to prevent self-fertilization which has led to more diversification between the species. The size of male gametophytes has reduced to a greater extent. The smaller pollen size reduces the period between the pollination due to the reduced distance, i.e. pollen grain reaching the female plant which ultimately leads to fertilization. After fertilization, the endosperm is formed before the first division of the zygote. The endosperm is the highly nutritive tissue that provides nourishment for the developing embryo and cotyledons.

Note: The presence of flowers helps in the fertilization of the ovule and formation of fruit consisting of seeds. The development of embryo and endosperm occurs inside the embryo sac, the sac wall increases in size and combines with the nucleus and the integument to form the seed coat. The wall of the ovary develops to form the fruit or pericarp, whose structure is closely related to the type of seed dispersal system.