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
In Lab 13 (“Flowers”), we looked at Fern fronds. The large fronds are the sporophyte generation of the fern, and underneath the fronds, in a linear tribal-face-paint-like pattern that would make any burning man participant jealous, are the sori. Each sorus is a group of immature sporangia that will eventually undergo meiosis and give rise to the gametophyte generation. Under the dissecting scope, the sori are clearly seen to be masses of sporangium, covered by a transparent indusium. We also looked at a variety of flowers. We identified the different parts of a flower and different forms a flower can take. Flowers can be classified as perfect (possessing both male and female sex organs) or imperfect (possessing either male or female sex organs), and complete (possessing all parts of a flower) or incomplete (missing a few parts) As a wise woman once said (cough.. Dr. L.-P…), flowers show us that it is possible to be perfect AND incomplete (cue group “Awww”). Images below: LEFT: Helleborus flower, dicot, perfect and complete. Picture taken by Katie Pardee MIDDLE: Cross-section of carpel, superior ovary (other flower organs are attached below the ovary). Picture taken by Jena Ozenna. RIGHT: Cross-section of the Alstromeria flower, monocot, inferior ovary (other flower organs are attached above the ovary). Picture taken by Jena Ozenna. -Katie Pardee
Submission by Jena Ozenna In lab this week, the main focus was flowering plants in comparison to the sporangia of mosses and ferns. To further understand the anther of angiosperms, we viewed prepared cross-section slides of Lilium anthers at both young and mature stages. Additionally, we observed the Wisconsin Fast Plants at 14 days of growth. The Wisconsin Fast Plants were planted on 9 February 2017. The plants were not pollinated at this point due to the stage of development.
Submitted by Lucas Maria Keoni Letelier The diversity of Oryza sativa L. is an oft forgotten yet definitive human concern. Our partnership with this group, lasting thousands of years, has seen the drawing out of the best qualities of the plant and the development of some of the more regrettable practices in the animal. Now, our actions have restricted our ability to cultivate some of our more popular varieties of rice and we must turn to obscure strains in order to develop new breeds combining high yields with salt and drought tolerance. In a warming planet, where unsustainable agricultural practices are steadily salting our soil, these qualities could expand our food production and provide one solution to an ever-looming, multi-faceted food crisis. In order to select the correct candidate in this breeding project, it is important to have a solid understanding of the traits that we are seeking out. It is one thing to understand that a plant does well in submerged or saline conditions, it is a different thing all together to know and understand the morphology that allows for this. Enter our class and seven rice varieties, three of which were breeding lines (Nipponbare, IR29 and N22) and four were land races (Kalo gorah, Nona Bokra, Pokkali, and Talmugur). We divided these varieties among the class, and recorded several key features. Of the shoot, we photographed and recorded the length of ligules (Fig. 1), we then took cross sections of the main blade 15cm, 10cm, and 5cm above the collar as well as 5 cm below the collar. These cross sections were stained with toluidine blue and the main veins were counted, as well as the number of aerenchyma chambers, occurence of silica in these, presence of sclerenchma along the abaxial side, and the occurence of abaxial bulliform cells. An example of one such cross section can be seen in Fig. 2. The roots were treated similarly; first the length from the bottom of the pot to the tip of the root was recorded, then the roots were counted and a representative was removed for cross sectioning. In order to understand the development of aerenchyma cavitation in maturing root tissues, cross sections were prepared from the bottom of the pot, 5cm, 10cm, 15cm, and 20cm from the bottom and 2cm from the tip of the root. These were also stained with TBO (Fig. 3) and proportion of aerenchymatous cortex was recorded with a characterization of clustered or diffuse arrangement, where possible. This was likely the most difficult preparation for the Pokkali group as the aerenchyma developed very quickly in these, already being visible in ~80% of tissue at the youngest section (nearest to the tip). At times, it felt like trying to cross section a plastic straw, with lots of splitting and flattening as we went. This data was all compiled online and will be used to elucidate the connections between these traits and the survival of the rice varieties under adverse conditions. Fig 1. The collar and ligule of the Pokkali variety, notice the absence of auricles. (Credit: Alex Abair) Fig 2. A cross section at 5 cm above the collar of a Pokkali, with silica crystals. notice the asymmetrical lengths of the blade on wither side of the "midrib" (top and bottom). (Credit: Alex Abair) Fig 3. A cross section from the middle of the root (~15 cm from the bottom of the pot) notice the well developed empty spaces within the cortex. (Credit: Alex Abair)
Capsules of unidentified moss photographed at 40x magnification by Alex Abair. The first stage (left image) shows an immature capsule with a sheath called a calyptera. If you find a capsule that’s a little more mature (middle image), it will have lost the calyptera, and an operculum will be visible. This operculum functions as a cap that keeps the spores contained in the capsule. If you find a capsule that’s even more mature than the previous two (right image), it will have lost both its calyptera and its operculum. There will be an opening at the end of the capsule lined with teeth. These teeth are collectively called the peristome, and they regulate the gradual release of spores.
-Alex Abair.
Last Thursday on February 16th, we observed cells undergoing mitosis on onion root tips. This was accomplished by a lengthy process that basically consisted of our instructor giving us "fixed" root tips, which is where cells are actively dividing. They are fixed because mitosis was suspended in the root apical meristem, or the root tip. We took these fixed root tips and put them in a dish labelled "Carnoy Fixative I" for 4 minutes. After 4 minutes, we transferred the root into a dish labelled "Carnoy Fixative II" for an additional 4 minutes. We then cut off about 1 to 2 mm of the root tip and put it on a slide. Once on the slide, we followed typical slide procedure and stained it with TBO. However, when we put the cover slip on the slide, we made sure to press straight down without twisting the cover slip. We then observed the slide under a compound microscope using oil immersion. This is what we saw: -Adalberto Marquez
Posted by Amber Eaton.
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