Gymnosperm 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.

Objectives

  • Compare and contrast angiosperm and gymnosperm life cycles.
  • Recognize the life cycle stages visible in the gymnosperm specimens in lab and explain what these stages do.

Specimens: whole

  • Male and female pine cones

Specimens: microscope slides

  • Pinus mature pollen grains
  • Pinus ovulate cone 1st year l.s.
  • Pinus staninate cone l.s.
  • Pine young ovulate cone l.s.

Ovulate cones

Start with the whole cones before you move on to the microscope slides. The ovulate cone, or female cone, is what most people think of when they think of a pine cone. It's large and woody, and it eventually produces seeds.

In longitudinal section, you can see the scales attached to the central axis. Each scale contains a sporangium (called a megasporangium in the female cone). Within each sporangium, meiosis occurs to make haploid spores. One spore is much larger and become a megaspore, which can then divide to form a female gametophyte. Compare this image to The Life Cycle of a Pine, fig. 30.4 in Campbell Biology (10th ed.).

Pinus ovulate cone with sporangium

Staminate cones

Pine staminate cone, longitudinal section

Staminate cones, also called male cones, produce pollen. Each cone contains numerous scales, each of which contains a sporangium in which pollen develops. First, cells in the sporangium undergo meiosis to produce haploid microspores. The bulk of the cone is diploid sporophyte tissue, but the pollen grains are individual haploid gametophytes.

Pollen

Pine trees, unlike lilies, are mostly wind pollinated. They produce vast amounts of pollen, which increases the likelihood of succesful fertilization. The individual pollen grains have winglike projections, which may help the pollen to get dispersed.

Pine staminate cone with pollen in sporangium

Pine seed with embryo

Pine seed with embryo

This is a longitudinal section of a pine seed. The embryo, a young sporophyte, is visible with its future leaves facing left. The embryo is surrounded by its food reserves within the seed. In gymnosperms such as this pine, the food reserves are haploid tissues of the female gametophyte. In angiosperms, the food reserves are triploid endosperm. The seed coat is missing in this specimen.

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