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THE SECRET LIFE OF PINE : Animation description


Three animations of increasing complexity are available on a VHS video tape or DVD covering the reproductive life cycle of a typical pine tree: 
1. Basic. Short, no terminology and easy to understand. (Running Time, 15m: 17s) 
2. Intermediate. Full length, with summaries and simple terminology. (Running Time, 19m: 30s) 
3. Advanced. Full length with scientific terminology. (Running Time, 16m: 46s)

All plants have what is known as an 'alternation of generations'. In pine, there are two plants: the large, familiar pine tree which is a spore-producing plant (sporophyte) and a very small gamete-producing plant (gametophyte). The pine life cycle

The diploid spore-producing plant (sporophyte)

A pine tree bears both pollen cones and ovule cones. Spore cases (sporangia) are found inside each type of cone.

Pollen formation: 
This occurs in the early spring. The sporangia of pollen cones (microsporangia) are in pairs on the lower surface of the cone scales and contain special cells called sporocytes. Sporocytes undergo meiosis, halving the number of chromosomes to produce numerous haploid microspores. As a result of meiosis, the chromosomes in the microspore cells are likely to contain new combinations of genetic material. Unlike mosses and ferns, the microspores are not released from sporangia in pine or in any other seed plant. Each microspore cell inside a microsporangium divides several times to form a young male gametophyte (several nonfunctional prothallial cells; a tube cell ; a generative cell), while the wall of each spore undergoes changes. The result is a pollen grain. The pollen grain is released from the sporangium in the spring of the first growing season.

The ovule cone and pollination: 
The sporangia of ovule cones (megasporangia) are located within the ovule. There are two ovules on the upper surface of each cone scale. Only one megaspore cell matures inside a megasporangium and this divides to form the female gametophyte which is kept inside the ovule. Pollen, carried by the wind, comes into contact with the ovule cones and some is forced in between the open cone scales. Inside the cone, the pollen falls downward and some comes in contact with a moist secretion (pollination drop). This fills a pore (micropyle) which leads inside the ovule. At this time, the cone scales enlarge and firmly join together, sealing the ovules and pollen inside. The pollination drop recedes, pulling the pollen down the pore and inside the ovule to the surface of the megasporangium.

The mature haploid gamete-producing plants (gametophyte)

The male and female tissue: 
Once on the megasporangium surface, the pollen grain germinates, forming a tube containing the tube cell. This pollen tube proceeds to grow into the megasporangium. The generative cell within the pollen grain divides to form a sterile and a fertile (spermatogenous) cell, and these move into the growing tube. During this time, the single fertile megaspore within the ovule divides to form a collection of nuclei representing the immature female gametophyte. Usually, the growing season comes to a close at this stage of development. In the spring of the next growing season, these nuclei form walls and several archegonia are formed. When the growing pollen tube locates an archegonium, the spermatogenous cell divides to form two sperm and discharges its contents. One sperm fertilises the egg cell. As there may be several archegonia, several eggs may be fertilised but each will require a pollen grain with pollen tube. 

Embryo and the seed formation: 
Each fertilised egg will form four genetically identical embryos that are nourished by the surrounding female gametophyte. All the embryos compete for space and nutrition but only one usually survives. This is repeated within adjacent archegonia if they were fertilised, so that multiple embryos form within a single female gametophyte. However, only one of these enlarges to form an embryo pine plant. This consists of several seed leaves (cotyledons), a shoot and a root tip and a root cap, and is surrounded by nutrient-rich female gametophyte tissue which in turn is surrounded by a tough seed coat. This structure is called the seed and is now ready for release. The seed contains an embryo pine tree with a supply of nutrients and is held in a resting (dormant) state. It is an important dispersal agent, able to be transported great distances and to withstand adverse conditions.

Seed germination

Upon the uptake of water, food reserves in the female tissue are made available for embryo growth. The root emerges, followed by the lower portion of the embryo stem known as the hypocotyl (because it is located below the cotyledons). Elongation of this stem pushes the seed up and out of the soil. At this point, the seed coat is shed and the cotyledons expand and become photosynthetic. Continued growth now is the result of activity in both the shoot and root apices.

For further information

The following books include a discussion of the pine life cycle:

  • Gifford, E.M. and Foster, A. S., Morphology and Evolution of Vascular Plants, 3rd ed., Freeman Publishers, 1989
  • Moore, R., Clark, W. D., and Vodopich, D. S., Botany, 2nd ed., WCB/McGraw-Hill Publishers, 1998.
  • Raven, P., Evert, R. and Eichhorn, S., Biology of Plants, 6th ed., Freeman-Worth Publishers, 1999.
  • Roost, T.L., Barbour, M.G., Stocking, C. R., and Murphy, T.M., Plant Biology, Wadsworth Publishing 1998