Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706 Embryology Lectures 14: Micro and mega-sporangium

in nonvascular plants gametophyte is dominant, sporophyte is visible in seedless vascular plants, sporophyte is dominant, gametophyte is visible in seed plants – gymnosperms and angiosperms, sporophyte is increasingly dominant, gametophyte is enclosed in the sporophyte

homosporous plants one kind of sporangia; one kind of spore; one kind of gametophyte; each gametophyte can produce both eggs and sperm

megasporangia megaspores mega-gametophytes Seed Plants are heterosporous plants: have two kinds of sporangia megasporangia megaspores mega-gametophytes microsporangia microspores micro-gametophytes * megagametophytes produce eggs; microgametophytes produce sperm

Megasporangia, megagametophytes, and eggs megasporangia are located within larger ovules - each ovule consists of integuments, a nucellus (the megasporangium), and a funiculus micropyle

produces megaspores enclosed in the ovule megasporogenesis - one or more cells in the nucellus (megasporocytes) undergo meiosis; produces megaspores enclosed in the ovule megasporocyte (haploid) megaspores (diploid) nucellus (diploid integument (diploid)

produces a megagametophyte (multicellular, haploid) - megaspore germinates in the nucellus, produces a megagametophyte (multicellular, haploid) - megagametophyte will produce at least one egg (haploid) (the process = megagametogenesis) egg megagametophyte nucellus integument

Microsporangia, microgametophytes, and sperm microsporangia consist of sporogenous tissue (microsporocytes) and a microsporangium wall

Sporangium sporangium microsporocytes (2n) microspores (1n) meiosis Sporangium sporangium

microgametogenesis: microspore (1n) microgametophyte germination (mitosis) - the microgametophyte is very small and enclosed in cell wall of a single microspore = a pollen grain

cells of the sporangium wall have diploid nuclei MICROSPORANGIUM microsporocytes (cells of the sporogenous tissue) have diploid nuclei microspores each have a haploid nucleus each pollen grain is made up of a few cells, each with a haploid nucleus each pollen grain (microgametophyte) produces at least two sperm and carries them to an ovule for fertilization

microsporangium opens to release pollen grains = dehiscence pollen grains are dispersed, landing near an ovule = pollination pollen grain germinates, producing a pollen tube

pollen tube grows to the egg cell; sperm travel through pollen tube, are released near egg egg megagametophyte nucellus integument ** POLLEN TUBE ELIMINATES THE NEED TO SWIM THROUGH EXTERNAL WATER

- fertilization of egg zygote (unicellular, diploid) zygote embryo (multicellular, diploid) embryo is enclosed in the ovule embryo (2n) megagametophyte (1n) nucellus (2n) Integument 2n

hardening of the integuments produces a seed - hardened integuments = seed coat - interior contains: the embryo (offspring sporophyte), and nutrition (stored in nucellus or other tissue) seed coat embryo nutrition seed

seeds provide many adaptive advantages to plants: 1) protection 2) carries nutrients 3) offspring dispersal

STAMEN, MICROSPORANGIUM AND POLLEN GRAIN Microspores matures into pollen grains Pollen grains, which contribute the male gamete, are form within an anther Anthers may be monothecal (two lobes or sporangia) or mostly bithecal (four lobes or sporangia)

Anther type Bithecal Anther

Pollen producing reproductive organ of flower. STAMEN Pollen producing reproductive organ of flower. They consists of a stalk – filament and anther which contains microsporangia. Most anthers are 2 lobed, attached to filament at base or to the center. Sterile tissue between lobes – connective. Anther has 4 microsporangia

Anther has 4 microsporangia. This forms sacs in anther. Each microsporangium lined with nutritive tissue - Tapetum, it contains diploid pollen mother cells.

Flower and its parts

These cells undergo meiosis to form haploid spores. Spores remain attached to each other or separate Each microspore divides mitotically to form immature microgametophyte called as – pollen grain.

Pollen released through opening of anther Pollen released through opening of anther. Pollen is carried by external agent like wind, water, insects etc and are pollinated. After pollination, pollen continues it's development inside pollen tube. Undergoes mitosis to produce 2 male gametes.

REMEMBER - Stamens in flower collectively called – Androecium REMEMBER - Stamens in flower collectively called – Androecium. It forms whorl around surrounding gynoecium. Column formed by fusion of multiple filaments is called – androphore. Some flowers have both male and female sexual organs. Some don't. Flowers with only stamens – androecious. Flowers with only carpels – gynoecious

MICROSPORANGIUM It's a sporangium that produces spores which give rise to male gametophytes. Microsporangium produces microsporocytes. It is also called as microspore mother cell. This creates 4 microspores through meiosis. Microspores divide to create pollen grains.

POLLEN GRAINS Pollen is fine to coarse powder containing microgametophytes of seed plants which produce male gametes. Pollen grains have hard coat – protects the sperms cells. When pollen is on the pistil, it germinates to produce pollen tube. Tube transfers sperm to ovule.

Pollen not a male gamete. Each pollen has vegetative and generative cell. This contains 2 nuclei - 1) Tube nucleus – produces pollen tube. 2) Generative nucleus – divides to form 2 sperm cells. Pollen produced in microsporangium. They come in different sizes.

Anther composed of mass of cells – it is undifferentiated. As flower develops 4 groups of sporogenous cells develop within anther. Fertile cells surrounded by sterile cells that grow into wall of pollen sac. Some cells grow into nutritive cells that provide nutrition. Microsporogenesis – Process by which 4 haploid microspores are produced from each diploid sporangenous cell after meiotic division.

After formation of 4 microspores, development of pollen grain wall begins. Pollen wall protects sperm nucleus. Pollen grain surface covered with wax and proteins which are held in place by structures called as sculpture elements. Outer pollen wall prevents pollen from shrinking and crushing the genetic material.

Pollen germination

Questions are welcome