Crystallized vitamin A
40x
Polarized Light
Stefan Eberhard
Doxorubin
Doxorubin in methanol and dimethylbenzenesulfonic acid 80x
Polarized Light
Lars Bech
Polarized Light Microscopy
Welcome to our website devoted to the exciting field of polarized light microscopy. Our intent of this site is to provide you informative articles and information on equipment for use in polarized light microscopy. The petrographic microscope, as it is often called, is used to view rock and mineral specimens under polarized light. Birefringent specimens will exhibit stunning colorations when subjected to polarized light filters. When these filters are crossed under a condition called cross polarization (also called crossed nicols), characteristics of the mineral specimen can be compared to known characteristics in rock and mineral charts for an accurate identification of the rock or mineral.
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The components of a polarized microscope consist of a Bertrand lens, the first polarizing filter called a polarizer, a second polarizing filter called an analyzer, and various compensator plates. The three most common compensator plates are the quartz wedge, gypsum, and mica plates. It is advised to take a course in optical mineralogy such as would be given in a geology course to gain an indepth understanding of the principles of a petrographic microscope and its application of rock and mineral identification. If you have a need for a petrographic microscope for the study of polarized light microscopy, you are encouraged to contact us about our wide selection. We can provide monocular versions, binocular, as well as trinocular polarizing light geological microscopes.
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Cell nuclei of the mouse colon
Cell nuclei of the mouse colon
740x
2-photon fluorescence
Dr. Paul Appleton
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Scanning electron microscope image of a leaf from a Black Walnut tree. Image shows a cross-section of a cut leaf, itsupper epidermal layer, mesophyll layer with palisade cells and vascular bundles, and lower epidermal layer. The protrusion at center is just over 50 microns tall. (Dartmouth Electron Microscope Facility/Dartmouth College
Grains
Pollen from a variety of common plants: sunflower, morning glory, hollyhock, lily, primrose and caster bean.
The largest one at center is nearly 100 microns wide.
(Dartmouth Electron Microscope Facility/Dartmouth College)
Pollen is a fine to coarse powder consisting of microgametophytes (pollen grains), which produce the male gametes (sperm cells) of seed plants. A hard coat covering the pollen grain protects the sperm cells during the process of their movement between the stamens of the flower to the pistil of the next flower.
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The structure of pollen
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Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell containing two nuclei: a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose cell wall and a thick, tough outer wall made of sporopollenin.
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Pollen is produced in the microsporangium (contained in the anther of an angiosperm flower, male cone of a coniferous plant, or male cone of other seed plants). Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see Electron micrograph at top right). Most, but certainly not all, are spherical. Pollen grains of pines, firs, and spruces are winged. The smallest pollen grain, that of the Forget-me-not (Myosotis spp.), is around 6 µm (0.006 mm) in diameter. Wind-borne pollen grains can be as large as about 90-100 µm.[
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The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archeology, and forensics.
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In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells form with in the anther, the fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac, some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. Four haploid microspores are produced from each diploid sporogenous cell called microsporocytes, after meiotic division. After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is made up of a resistant compound called sporopollenin; the intine is made up of cellulose and pectin. The exine is what is preserved in the fossil record.
Pollen grains may have furrows, the orientation of which (relative to the original tetrad of microspores) classify the pollen as colpate or sulcate. The number of furrows or pores helps classify the flowering plants, with eudicots having three colpi (tricolpate), and other groups having one sulcus.
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Except in the case of some submerged aquatic plants, the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.
Snow Crystal
Rime on a columnar snow crystal.Contact between the snow crystal and the supercooled droplets in the air resulted in freezing of the liquid droplets onto the surface of the crystal.
Observations of snow crystals clearly show cloud droplets measuring up to 50 microns on the surface of the crystal.
(Agricultural Research Service, United States Department of Agriculture)
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A Snowflake Primer
... The basic facts about snowflakes and snow crystals ...
Snowflakes and snow crystals
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Snowflakes and snow crystals are made of ice, and pretty much nothing more. A snow crystal, as the name implies, is a single crystal of ice. A snowflake is a more general term; it can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals that form "puff-balls" that float down from the clouds.
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The structure of crystalline ice
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The water molecules in an ice crystal form a hexagonal lattice, as shown at right (the two structures show different views of the same crystal). Each red ball represents an oxygen atom, while the grey sticks represent hydrogen atoms. There are two hydrogens for each oxygen, so the chemical formula is H2O. The six-fold symmetry of snow crystals ultimately derives from the six-fold symmetry of the ice crystal lattice.
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Snowflakes grow from water vapor
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Snowflakes are not frozen raindrops. Sometimes raindrops do freeze as they fall, but this is called sleet. Sleet particles don't have any of the elaborate and symmetrical patterning found in snow crystals. Snow crystals form when water vapor condenses directly into ice, which happens in the clouds. The patterns emerge as the crystals grow.
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The simplest snowflakes
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The most basic form of a snow crystal is a hexagonal prism, shown in several examples at right. This structure occurs because certain surfaces of the crystal, the facet surfaces, accumulate material very slowly (see Crystal Faceting for more details). A hexagonal prism includes two hexagonal "basal" faces and six rectangular "prism" faces, as shown in the figure. Note that a hexagonal prism can be plate-like or columnar, depending on which facet surfaces grow most quickly.
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When snow crystals are very small, they are mostly in the form of simple hexagonal prisms. But as they grow, branches sprout from the corners to make more complex shapes. Snowflake Branching describes how this happens.
The Morphology Diagram
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By growing snow crystals in the laboratory under controlled conditions, one finds that their shapes depend on the temperature and humidity. This behavior is summarized in the "morphology diagram," shown at left, which gives the crystal shape under different conditions. Click on the picture for a closer view. The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions. For example, we see that thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F). Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F). Furthermore, we see from the diagram that snow crystals tend to form simpler shapes when the humidity (supersaturation) is low, while more complex shapes at higher humidities. The most extreme shapes -- long needles around -5C and large, thin plates around -15C -- form when the humidity is especially high.
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Why snow crystal shapes change so much with temperature remains something of a scientific mystery. The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. It is the subject of current research in my lab and elsewhere.
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The life of a snowflake
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The story of a snowflake begins with water vapor in the air. Evaporation from oceans, lakes, and rivers puts water vapor into the air, as does transpiration from plants. Even you, every time you exhale, put water vapor into the air. When you take a parcel of air and cool it down, at some point the water vapor it holds will begin to condense out. When this happens near the ground, the water may condense as dew on the grass. High above the ground, water vapor condenses onto dust particles in the air. It condenses into countless minute droplets, where each droplet contains at least one dust particle. A cloud is nothing more than a huge collection of these water droplets suspended in the air. In the winter, snow-forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing. The water is said to be supercooled, meaning simply that it is cooled below the freezing point. As the clouds gets colder, however, the droplets do start to freeze. This begins happening around -10 C (14 F), but it's a gradual process and the droplets don't all freeze at once. If a particular droplet freezes, it becomes a small particle of ice surrounded by the remaining liquid water droplets in the cloud. The ice grows as water vapor condenses onto its surface, forming a snowflake in the process. As the ice grows larger, the remaining water droplets slowly evaporate and put more water vapor into the air. Note what happens to the water -- it evaporates from the water droplets and goes into the air, and it comes out of the air as it condenses on the growing snow crystals. As the snow falls there is a net flow of water from the liquid state (cloud droplets) to the solid state (snowflakes). This rather complicated chain of events is how a cloud freezes.
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The rest of the story
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Alas, there's so much more to the story -- it simply cannot fit here on a single page. Snowflakes are fascinating objects (in my humble opinion), and you can learn all kinds of interesting things about them in The Snowflake: Winter's Secret Beauty. Click here to see what's inside this book.
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Very comprehensive site and beautiful photos of snow crystals
http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm
Source of image
http://www.boston.com/bigpicture/2008/11/peering_into_the_micro_world.html
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