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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If the title did not make it apparent, this post is all about wood! Part of the lab activities this week included the examination of woody tissue. Before I get into the really cool stuff, I am going to introduce some terminology. There are various ways that a cut can be made through a piece of wood, and each one gives a different perspective of the internal structures. The main types of cuts are the cross-section, radiate section, and tangential section. Here is a quick breakdown of the cuts, and what makes them unique. The cross-section is a complete cut through a “stem” at a right angle from its axis (the longitudinal surface of the stem). This stem can be as small as a twig or as large as an entire tree trunk! The radiate section is a longitudinal cut through the center of the stem, and the tangential section is a longitudinal cut that does not go through the center. Examples of all these cuts can be seen in Figure 1.1 below. Figure 1.2 (see below) shows one of the great “tree cookies” we had available to examine during lab. It allowed for us to view all of the essential parts of a tree, both living and dead. This image is labeled to show the various components and tissue systems that make up the trunk. The tree in the image is ring porous. This means the annual growth can be seen as rings. The cells of this region (xylem) carry water and nutrients from the roots to the leaves. These cells get very large during rainy seasons to compensate for the increased water flow. Therefore, these cells are also smaller during the dry seasons, and this combination yields the rings seen in the cross section. By counting the rings, I estimate this tree was ~42 years old when it fell. I want to make another point with Figure 1.2 before moving on. The cross-section shown is ~55cm in diameter. The section of the trunk labeled “c”, the periderm & phloem, is maybe 3cm across. What is amazing is that this section is the only “living” part of the trunk! The cells that make up sections “a” and “b” are actually dead when mature, making the majority of the tree dead! Before concluding, I want to show some more detail into what “ring porous” is all about. I briefly stated earlier that the size of the water conducting cells change as the amount of rainfall changes. Figure 1.3 is a great example of how this looks at under the microscope. This figure shows a cross-section of the secondary xylem of Quercus or Oak. It is very easy to differentiate the early and late wood by the size of the cells, with early wood being much larger. This is due to the large amounts of water and nutrients available in the Spring, or beginning of the season. With these microscope slides it is easy to see how the rings of a tree form! I hope you learned something. Cheers! Post by Michael Belcher This may seem like a silly question but it is not without answer. Of course in this case we are referring to the plant Capsella bursa-pastoris, commonly referred to as the shepherd's purse. This dicot earned its name because of the odd, slightly purse shaped, fruits that it produces. These fruits contain two chambers and are sometimes referred to as siliques. Each fruit contains several seeds which have to ability to remain dormant for long periods of time but have a short germination time. So to answer the initial question, Shepherd's need a purse to carry their seeds. Figure 1. Capsella bursa-pastoris with visible flowers and fruits. Image courtesy of blog.emergencyoutdoors.com. The flowers of this plant are small and white (See Fig. 1) and contain four petals and six stamens. This plant is gathered from the wild or cultivated to make food, medicine, and cosmetics. As mentioned previously, each fruit contains several seeds. Within each of these seeds is an embryo that is the source of life for future generations of the plant. Coincidentally the as the young embryo forms if reaches a stage when it resembles the heart shaped fruit that contains it. Figure 2. A diagram of the seed cross section as it develops. Diagram illustrated by Melissa Owens In the second image in the diagram the heart shaped embryo can be seen. The lobes of this heart shape with eventually develop into the cotyledons of the future plant. Another plant with a unique fruit structures Lilium sp. Many people are familiar with lilies as they are a beautiful and fragrant flower often grown indoors or added to bouquets. The lily contains a superior ovary that is located above the point at which the anthers attach. Unlike the shepherd's purse seeds are not the only form of reproduction for lilies. They also form a large bulb underground that is used as an overwintering structure. Some also from rhizomes that can produces more plants. A diagram of the lily ovary can be seen in Figure 3. Figure 3. Diagram of Lilium sp. ovary cross section. Diagram illustrated by Melissa Owens.The diagram illustrates the orientation of the ovule and the structures it contains. Here the placenta can been seen in the center. This is the source of nutrients for the developing seed. Each ovule is attached to the ovary by the funiculus. This connection will eventually be severed once the seed reaches maturity. Figure 4 (below) shows a more detailed image of the lily fruit cross section. Figure 4. Lilium sp. fruit cross section. Prepared and photographed by Melissa Owens at with TBO stain at 40X. It is amazing how every plant forms a different type of fruit to house it's seeds. Even though they may appear completely different, each contains the same fundamental structures that will form into a new plant after the seed has germinated.
Post by: Melissa Owens Is your mind blown yet? No? Well then let's continue the exploration into the blatant identity theft of fruits. In our Plant Structure Lab this week, we examined a plethora of fleshy and dry fruits. Using the "Key to Common Fruits" handout, our job was to identify the imposters with misleading common names. For example, that ballpark favorite, the peanut, isn't actually a nut at all. It is a legume in the family, Fabaceae. That's right, the same family as the broad bean we looked at in earlier labs. This peanut has marginal placentation because the ovules develop in rows near the margin on the placenta formed along the ventral suture. The pericarp of the peanut is the dehiscent outer shell that we crack to get to the yummy fruit inside. It is a fusion of the exocarp, mesocarp, and endocarp. Moving on to the fleshy fruits, we dissected a beautiful, juicy, orange. In the orange, or Citrus sp., we were able to see clearly the exocarp, endocarp, and mesocarp. The endocarp of the orange is chock full of juice-filled hairs that are used for food storage for the embryo. The mesocarp has many oil filled sacs, from which that wonderful citrus smell comes from. The exocarp is the outer peel that protects the embryo and endosperm. Surprise, surprise, the simple fruit of the orange turned out to be a type of berry, called a hesperidium. The orange shows axile placentation because the placentae develop from the central axis which corresponds to the confluent margins of carpels. On Thursday in lab, we examined a series of cross-sections of Tilia stems of different ages, looking for signs of secondary growth. Secondary growth occurs in most seed plants, like dicots and gymnosperms, but very rarely in monocots as their vascular tissue cannot form into a ring. The lateral meristems, vascular cambium and cork cambium, are responsible for the production of secondary xylem and secondary phloem. In woody plants, this produces wood and bark. In non-woody plants, secondary growth can result in thickened, modified stems such as potatoes. Now my challenge to you: can you name what types of fruits the following are? Are they dry or fleshy? Dehiscent or indehiscent? What type of placentation, locule number? Blog posted by: Holly Giorgio-Dundon
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