Much of this class has been devoted to learning about angiosperms and more general plant structures, but this week we finally got to take a closer (though brief) look at some of my favorite plants - gymnosperms, specifically Pines. As successful as angiosperms are, gymnosperms can also be found everywhere and thrive in even the most rugged and challenging environments. Looking first at the name ‘gymnosperm’ we can break it down into two greek roots - gymnos meaning ‘naked’ and sperm meaning ‘seed.’ Gymnosperms vary most significantly from angiosperms in that their seeds are naked - that is, they lack the protective seed packaging that make up the bulk of an angiosperm’s fruit. Despite these differences, the overall reproductive cycle of gymnosperms is similar to that of angiosperms, so let’s take a closer look at how gymnosperms do this. In pines, the megasporangia and microsporangia are produced by separate cones on the same tree. The pollen from the pollen-producing, or microsporangiate cones is carried by the wind to the megasporangiate, or ovulate cones either on the same tree or more likely a different tree. The above slide is actually a pine cone, though it isd much smaller than the large, woody, scaled pine cones you may be familiar with. This is the microsporangiate male cone, and it’s only function is to produce pollen. These small orange cones can typically be found on the ends of branches in the spring. One cone will contain many microsporocytes producing 4-celled pollen grains. Below, you can get a closer look at a few of the microsporophylls (the larger oblong structures) full of mature pollen grains (very small, round structures). Next up are the female ovulate cones. Within these cones are the archegonia, which are small structures containing the female gametophyte. In the center of the slide you can see the two side-by-side egg cells within the archegonia. Just above these is the micropyle, barely visible for the left ovule. Just as in angiosperms, the pollen grains will grow pollen tubes through the micropyle to deliver the sperm nucleus to the ovule and form a zygote.Once fertilized, these embryos will grow into a complete seed within the ovulate pine cone. The ovulate pine cone is the woody, scaly, egg-shaped cone that everyone associates with pines. Each scale of the pine cone is called an oviliferous scale, and bears two seeds attached near the base. Once the seeds are fully mature they are released from their cones to go off on their own. The exact processes for this are as varied and fascinating as seed dispersal in angiosperms and as much as I would love to tell the stories of how different pines cast away their tiny seeds, this is where I must end. I hope you have enjoyed learning about gymnosperm reproduction as much as I have!
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AuthorContent is created by students participating in the Plant Structure course at Oregon State University for Winter 2017. Archives
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