Angiosperm life cycles

This page is part of Plants III. By this time you've looked at the life cycles of mosses and ferns (Plants I) and studied the tissue and organ structures of a variety of vascular plants (Plants II). In this lab, Plants III, you'll look at a variety of structures related to reproduction in the two groups of vascular seed plants: angiosperms and gymnosperms. As with the previous plant labs, you should try to interpret these structures in terms of the plants' life cycles, using the life cycle diagrams in Campbell Biology for guidance.

There are two pages on this site for Plants III: Angiosperm life cycles and Gymnosperm life cycles life cycles.

Life cycle review

All plants, like the rest of the eukaryotes, have a haploid and a diploid phase in their life cycle. In addition, all plants also show alternation of generations: both the haploid phase and the diploid phase are multicellular. This, of course, is quite different from the animal life cycle; in animals, the haploid phase is unicellular and consists only of an egg or a sperm.

Now would be a good time to review the moss and fern life cycles in Campbell and remind yourself of the fundamental things that all plant life cycles share:

  • Sporophyte: Diploid (2n) and multicellular. Cells in the sporophyte undergo meiosis, cutting chromosome number in half to make haploid spores.
  • Spore: Haploid (1n) single cell, produced by meiosis, that can grow into a new multicellular haploid phase called the gametophyte.
  • Gametophyte: Haploid multicellular stage; makes gametes via mitotic cell division.
  • Gametes: Haploid egg and sperm, produced by mitosis in fertilization, they fuse to form a dipoid zygote.
  • Zygote: Diploid single cell, grows to form sporophyte.

While all the plants show alternation of generations, they vary in terms of the size and degree of independence of each phase.

Nonvascular plants (e.g. mosses):

The gametophyte is large, leafy, and green (photosynthetic). The sporophyte is smaller and shorter-lived, and it grows as a brown stalk out of the female gametophyte. Moss gametophytes produce naked sperm that need water to swim to the egg in the female gametophyte. Mosses live on land, but they can only complete their life cycle in very wet conditions (drenched in rain, for example).

Vascular non-seed plants (e.g. ferns):

The sporophyte is large and green. The gametophyte (also green) lives separately but is small and short-lived. Like mosses, fern gametophytes make naked sperm that needs water to swim to the egg. Fern gametophytes commonly make both eggs and sperm, but since they make these at different times, fertilization usually occurs between two different gametophytes. Ferns also need very wet conditions to complete their life cycle.

Vascular seed plants, including both gymnosperms (e.g., pine trees) and angiosperms (flowering plants, e.g., lilies):

The sporophyte is large, long-lived, and green. The gametophyte is tiny (only a few cells) and develops completely inside the sporophyte. The male gametophyte is a pollen grain, which can withstand dry conditions. Seed plants can reproduce in dry conditions. What makes seed plants different? Seed plants are highly adapted to life on land. Non-seed plants such as mosses or ferns require water to carry their gametes for fertilization. Seed plants have two key tricks that allow them to reproduce in dry environments:

  • Seeds. Seeds provide protection and nourishment for the developing embryo (the young sporophyte). Thanks to seeds, plants can be dispersed widely by animals or by wind. Seeds also allow many plants to survive harsh conditions. In a vascular non-seed plant such as a fern, the young sporophyte is dependent on the gametophyte, so it can’t be dispersed and can’t survive a period of harsh weather that would kill the gametophyte.
  • Pollen. Pollen grains contain male gametophytes, the tiny haploid stage that makes the sperm. The pollen is encased in a tough outer coat that resists desiccation, and can be widely dispersed by animals or wind. Instead of having gametes (sperm) swim to their destination, seed plants get the whole male gametophyte (pollen) to the place where fertilization must occur. Then the pollen can release sperm to fertilize the egg. In both conifers (pine trees) and flowering plants, the sperm is only a nucleus carried within a pollen tube; there is no swimming sperm. The female gametophytes remain within the sporophyte, inactive until the pollen arrives.

Angiosperms

Angiosperms (flowering plants) account for most of the diversity of plant life. Their ability to live in a wide range of habitats is due, in part, to the fact that they are vascular seed plants, with the ability to grow and reproduce in wet or dry conditions. In addition, angiosperms have extensively coevolved with various animal species in ways that aids angiosperm reproduction. Animals play key roles in both pollination and seed dispersal for many angiosperms, and this is due to the two unique features of angiosperms: flowers and fruits. Flowers in many species attract pollinating animals such as insects. Fruit, which is actually the swollen wall of the ovary, attracts other animals to eat the fruit and disperse the seeds it contains.

Objectives

  • Compare and contrast the life cycles of mosses, ferns, angiosperms, and gymnosperms. Describe how the life cycles of seed plants enable reproduction in dry conditions.
  • Identify the life cycle stages shown in all the specimens we have in lab and say what these stages do.

Specimens: whole

  • Lily flowers and fruits (whole & fresh)

Specimens: microscope slides

  • Lilium anthers pollen tetrads c.s.
  • Lilium pollen w.m.
  • Lilium pollen tubes style l.s.
  • Lilium mature seed embryo c.s.
  • Lilium ovary general structure c.s.

Whole flowers & fruits

We'll use lilies as the main example of angiosperm life cycles, partly because there are lilies planted in the beds outside the classroom. Lily flowers, like many other flowers, contain both male and female reproductive parts. Lilies are monocots; like other monocots, they have flower parts in multiples of three: six stamens and an ovary divided into three compartments.

Start by looking at whole flowers before you look at the microscope slides. Compare the flower to fig. "The structure of an idealized flower," 38.2 in Campbell Biology (10th ed.). Identify these structures in the lily flower:

  • Sepals (3)
  • Petals (3)
  • Carpel (1): the female part of the flower, including the ovary, style, and stigma. The ovary is where female gametophytes are produced; these female gametophytes eventually produce eggs. The stigma receives pollen, usually carried by an insect.
  • Stamens (6): the male parts of the flower, each including an anther and a filament. Pollen is produced inside the anthers, and released when each anther splits open.

After fertilization, the ovary matures into a fruit. Examine the mature fruit of the lily flower and compare it to the ovary.

Once you've figured out the anatomy, compare it to "The life cycle of an angiosperms," fig.38.4 in Campbell Biology (10th ed.).

Lily flower cross section

Slide: Lilium anthers 2nd division cross section

Lily flower cross section with ovary and anthers

Once you have a good idea of the overall anatomy of the flowers, look at this slide. It is intended to show the anthers, but it is actually a cross section of the entire immature flower, featuring the ovary in the middle surrounded by six anthers. The ovary is divided into three compartments, and each anther is divided into four compartments. Compare this to a whole flower and make sure you see how this slide is a cross section of that flower.

In the mature flower, the anthers are at the tips of the long filaments, far above the ovary. However, in this immature flower, one cross section shows both ovary and anthers in the same cross section.

Most of the tissue you see here is part of the sporophyte. Inside the ovary, spores will be formed that can grow to become female gametophytes. Inside the anthers, other spores will be formed that can grow to become male gametophytes, which become pollen.

Ovaries

Lily ovary, cross section

Most of the structure of the ovary is part of the sporophyte; it's diploid. In cross section you can see that the ovary is three-sided, and it contains numerous ovules. Within each diploid ovule, meiosis occurs, producing a haploid female spore (called a megaspore because it's large; the smaller male spore in the anther is called a microspore). The ovule is part of the sporophyte, but it will eventually contain the haploid gametophyte. This is one of the main things you should know about angiosperm life cycles: the gametophytes are microscopic and are formed completely inside the sporophyte. The megaspore eventually goes through three rounds of cell divison to form an 8-nucleus female gametophyte, also called the embryo sac. The gametophyte completes mitotic nuclear divisions but does not divide into 8 separate cells; later, it divides the large cell into seven cells containing the 8 nuclei. One of these cells becomes the egg, and the others play supporting roles in development.

As you look at this slide, you should recognize that not every cross sectional slice will show a female gametophyte inside each ovule; it's a matter of luck to slice it so that the female gametophyte is visible.

Lily ovary cross section with embryo sac

Lily ovary, higher magnification with gametophyte

Anthers & pollen

Lily anther, cross section

Anthers produce pollen. Each anther is divided into four compartments. The anther is part of the diploid sporophyte, but meiosis inside the anther to produce haploid spores, called microspores (microspores are male spores; megspores are female). After meiosis, each spore divides mitotically, eventually producing a single cell with two or three haploid nuclei. This multinucleate cell is the male gametophyte. The male gametophyte gains a tough outer wall to become a pollen grain.

Lily anther cross section, sporangium with dividing pollen

This image of dividing pollen is a magnified detail of the image above. It shows some pollen cells in the process of mitosis. Chromosomes are stained purple. In cells that are in the midst of mitosis, the individual chromosomes are visible as stringy structures; otherwise the chromosomes are decondensed so the nucleus appears as a single round blob.

Pollen

Germinating pollen

If a pollen grain lands on the stigma of another flower, it is likely to germinate. During germination the pollen grain forms a long pollen tube that may eventually carry two pollen nuclei into the ovary to fertilize an egg

Seed

Lily seed, cross section

In this cross section, the embryo is seen as a circle, corresponding to a cross section of the stem of the new sporophyte. The embryo is surrounded by triploid endosperm tissue, which in turn is surrounded by a diploid seed coat derived from the parent sporophyte. Thus, an angiosperm seed such as this contains three genetically distinct groups of cells.

References & further reading

Campbell Biology, Chapter 38, Angiosperm Reproduction and Biotechnology. In particular, study fig. 38.4, The life cycle of angiosperms, and the various diagrams of flower and fruit structure. See also Chapter 30, Plant Diversity II: The Evolution of Seed Plants.

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