Life Cycle of Pine Trees

Pine trees are gymnosperms, a group of plants that reproduce via "naked seeds.” They are more complex than the mosses because they have both phloem and xylem. They are more complex than the ferns which spread themselves via spores because this group of plants uses seeds to spread their descendants. However, they are less complex than the angiosperms which enclose their seeds within a vessel (known as the ovary). Gymnosperms seeds are known as the naked seeded plants because their seeds are exposed directly to the air when the seed cone opens up.

Continental Drift Image Source: “Plate Tectonics” article written for Encyclopedia Britannica by contributors Tjeerd H. van Andel and J. Brendan Murphy. Available at this web address: https://www.britannica.com/science/plate-tectonics

The ancestor of pine trees evolved in the northern supercontinent, Laurasia. Due to continental drift, pine trees are natively dispersed in North America, Europe, and Southeast Asia. Pine trees belong to the genus Pinus. They are evergreen and tall-growing gymnosperms. The trees are monoecious, producing both male and female cones on the same tree.

Pine trees prefer full sunlight and acidic or sandy soils. They can grow up to 150 feet and have a lifespan of more than 100 years. Pine trees have evolved adaptations to survive where other trees cannot such as high altitudes and rocky soils. They can grow in soils high in limestone.

The phases of the life cycle of pine trees include:

1. Strobilus

2. Pollination

3. Fertilization

4. Growth

Strobilus

Fully mature pine trees produce strobulii on their branches. We commonly call the strobulus a “pine cone.” Technically, they are sporophylls (spore-producing leaves) arranged in a spiral. Pine trees will produce male strobulii (or pollen cones) on their lower branches while female strobulii (known as seed cones) develop on the upper branches. Photo credit: “The Sex Life of the Christmas Tree,” by Patricia Hanbidge. Article available from the Battlefords News-Optimist at this site: https://www.newsoptimist.ca/opinion/columnists/the-sex-life-of-the-christmas-tree-1.23526750

The male cones consist of microsporophylls (both the prefixes micro and anther refer to male in Botany). The microsporophylls contain microsporangia which produce the microsporocytes. These microsporocytes go through meiosis to produce 4 haploid microspores that develop into immature male gametophytes known as pollen grains. The male gametophytes will not reach maturity until they produce sperm (much later). In the spring months, male strobili release pollen to the wind in the hopes of reaching female cones. The male cones shrivel and fall from the tree after pollen release. Photo sources: (top image) “How Will Pine Pollen Benefit You?” posted by LeighLon Surthrival at this web address: https://www.surthrival.com/blogs/news/pine-pollen-benefits (bottom image) “Pine Pollen MC” available at this web address: http://botit.botany.wisc.edu/Resources/Botany/Gymnosperms/Coniferophyta/Pine%20life%20cycle/Microsporangiate/Pollen/pine%20pollen%20MC.jpg.html

Female cones are typically larger than male cones and consist of spirally arranged megasporophylls (both the prefixes mega and archae refer to female in Botany) referred to as scales. Within ovules located at the base of the scales, megasporangia produce megasporocytes that undergo meiosis to produce 4 megaspores. Three of the megaspores degenerate, leaving one to slowly grow into the female gametophyte over several months. Each scale contains 2 ovules. The ovule is made of the nutritive nucellus and an integument that surrounds it. The nucellus will provide a food source to the growing gametophyte. The integument will become the seed coat. The integument has a tiny hole, the micropyle, which will allow entry of the pollen tube. Image source: “Gymnosperms” available at this web address: http://webprojects.oit.ncsu.edu/project/bio181de/Lab/plant_phylogeny/gymnosperms.html

Pollination

Seed cones usually take two growing seasons to fully mature and produce seeds. During the first spring, immature cone scales spread apart, allowing pollen grains to be blown into them. Pollen grains get stuck in sticky drops of fluid, known as pollen drops, that are oozing out of the micropyles. As the fluid evaporates, the pollen is drawn into the micropyle to the top of the nucellus. The female scales grow together to protect the developing ovule.

· Meiosis and megaspore development won’t occur until about a month after pollination.

· Once the functional megaspore is produced, the female gametophyte won’t mature until more than a year later!

Meanwhile, the pollen grain produces a pollen tube that slowly digests its way through the nucellus as it grows toward the archegonium. As it grows, two of the original four cells (the microspores) enter the pollen tube. One of the cells, known as the generative cell, divides to produce the spermatogenous cell and a sterile cell. The spermatogenous cell will divide and produce two male gametes, or sperm. At this point, the germinated pollen grain is a mature male gametophyte (because it has produced sperm).

Fertilization

About 15 months after pollination, the tip of the pollen tube reaches the archegonium, unites with it, and discharges its contents. One sperm fertilizes the egg, producing a zygote. The other sperm and pollen grains disintegrate. Sperms from other pollen grains can fertilize eggs of neighboring archegonia (similar to the formation of fraternal twins or triplets in animals). The zygotes develop into embryos that are nourished by the female gametophyte. As the embryos develop, a layer of the integument hardens and becomes the seed coat. A thin, membranous layer of the cone scale becomes a “wing” on the seed. Seeds are dispersed by the wind or wildlife, such as birds and squirrels.

Growth

In cool, temperate forests, pine sees require a dormancy period, in which they must be buried in leaf litter and experience both cold and moist conditions. If the seeds found themselves in suitable conditions (acidic or sandy soils), they will germinate. Pine species succeed in rocky areas and at high altitudes that would be too challenging for many tree species. They can grow up to 150 feet tall and live for over 100 years.

Works cited:

“Chapter 22: Introduction to Seed Plants: Gymnosperms.” Introductory Plant Biology, by James E. Bidlack and Shelley Jansky, McGraw-Hill, 2018, pp. 418–425.

“Chegg.com.” Definition of The Life Cycle Of A Pine Tree | Chegg.com, www.chegg.com/homework-help/definitions/the-life-cycle-of-a-pine-tree-14.

Davis, Skip. “Life Cycle of Pine Trees.” Hunker, www.hunker.com/12434739/life-cycle-of-pine-trees.

Eckenwalder, James Emory. “Pollination.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 May 2018, www.britannica.com/plant/conifer/Pollination.

Life Cycle of Gymnosperms

Pine Tree

Image modified from: Pearson Education