Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
conifers and allied divisions
This laboratory exercise continues the examination of vascular plants. In ferns the gametophyte generation, the prothallus, lived on the ground and produced eggs and flagellated sperm. The plants to be examined in this and future labs are called seed plants; the male gametophytes are called pollen and the female gametophytes, contained in ovules, are dependent on the sporophyte plant. After fertilization, an embryo sporophyte forms within the ovule; haploid tissue of the gametophyte and diploid tissue of the ovule integument form a tough capsule called a seed. Conifers are one division of a larger group called the gymnosperms, which means plants with naked seeds. The comparable descriptive term for flowering plants is the angiosperms; in them the seeds are in a container called a fruit. As you look at the materials, keep in mind the comparison of conifers with Marchantia and with ferns in terms of alternation of generations and the various structures. The conifers are entirely independent of water for fertilization of gametes.
The gymnosperms include 4 divisions: Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta. Ginkgo trees are widely planted in New York City, e.g., nearby in Madison Square Park, because they are resistant to pollution. The leathery, fan-shaped leaves are borne close to the stem and not from the tips of twigs. Trees are male or female.
The Pine tree that you see is the sporophyte. The needles are specialized leaves. Genus Pinus slides:
Pinus: two needle leaf c.s. The needles in most pines are retained for two to four years and must endure the ravages of freezing temperatures, drought, and pollution. The U-shaped cross section allows the needle to be twisted without buckling, whereas a cylinder would fail. The epidermis of a pine needle is covered with a thick cuticle and underlain by compact hypodermis cells. Recessed stomata perforate these layers and lead into air spaces in the mesophyll. Photosynthetic mesophyll cells are tightly packed and irregularly shaped; wall ridges project into the cells, giving them more surface area and adding strength to the needle. Resin ducts pass through the mesophyll (a duct can be seen on photo on left at near top on right side). Two vascular bundles are near the center of the needle; xylem is always toward the upper side (adaxial side), phloem toward the lower (abaxial side). The vascular bundles are surrounded by transfusion tissue that is believed to conduct materials between mesophyll and the vascular bundles; it is surrounded by endodermis, a single layer of cells.1
Pines are monoecious, both male and female cones or strobili (strobilus, singular) are borne on the same plant, but female cones are in the higher branches; this minimizes self-pollination. Megastrobili (female cones) are large and woody; microstrobili (male cones) are small and flexible. Slides:
Pinus: mature male strobilus. In the microstrobilus spirally arranged bracts bear sporangia on the underside or basal face (toward the base of the cone) The bracts are called microsporophylls and the sporangia, microsporangia. Inside of each microsporangium, microspore mother cells (2n) divide by meiosis to form haploid microspores (n). Each microspore cell divides by mitosis into a 4-celled gametophyte called a pollen grain.
Pinus: mature pollen grains w.m. Pine pollen (male gametophyte) has a pair of concave wings that will help it travel on the wind. Two cells are obvious in the body of the gametophyte: the tube cell will grow a tube to the egg in a female cone; the generative cell nucleus will divide into two nonflagellate sperm, and one of them will travel through the tube and fertilize the egg. Pine pollen is transported from male to female cones by the wind; there has to be a huge amount of pollen produced, since most of it is wasted.
Pinus: micropyle. This is a longitudinal section through a young megastrobilus or female cone. Megastrobili can take 2 to 3 years to mature. The spirally arranged bracts are woody and bear ovules on the upper or apical side (toward the tip of the cone). The bracts are called megasporophylls. An ovule is like a vase lying on its side with its mouth, the micropyle, facing the cone axis. The outer integument nearly surrounds the megasporangium. The megaspore mother cell (2n) inside the megasporangium undergoes meiosis into 4 haploid megaspores (n), 3 of which degenerate. The remaining megaspore divides by mitosis to produce the female gametophyte, and inside it archegonia develop and an egg forms in each one. Pollen must reach the micropyle to fertilize the egg. Sticky drops produced by the megastrobilus trap pollen.
Pinus: archegone l.s. This slide shows an ovule. The integument (2n) surrounds the female gametophyte (n), in which there are two archegonia; an egg will develop in each archegonium.
When fertilization has taken place, a diploid zygote (2n) forms. Following fertilization, cones close to protect the developing embryos. The zygote divides by mitosis to make an embryo sporophyte with parts that will become the root, stem, and leaves. The energy rich haploid female gametophyte that surrounds the embryo will be its food source during germination. Fertilization causes the integument of the ovule to harden into a seed coat. A true seed is thus a combination of tissues: The hard outside covering of integument is part of the parent sporophyte (2n); it surrounds the female gametophyte (n), which contains the germ of the next generation, the embryo sporophyte (2n). The cone opens and seeds, each with a thin vane, are dropped for scattering by the wind.
1Information about pine needles is from P. H. Raven, R. F. Evert, and S. E. Eichhorn, 1999, Biology of Plants, Sixth Edition, New York, W.H. Freeman and Co., p. 474-476.
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Last updated 22 September 2007 (JHW)