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Helianthus and Zea stems, transverse sections
Return to these slides for a moment. What is the difference between the two sections of Helianthus, apart from their size?
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How has this happened and why would it be impossible in the Zea stem? ___________________________________________________________________
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Of course sunflowers are not really trees and that is about as far as secondary growth goes in a herbaceous plant. To see further development we need to look at a true woody plant.
Tilia (Basswood) stem, prepared section
Use the X4 objective to get a general view of the section. Try to find periderm, phloem, cambium, xylem and rays. How do we know that this is a 3-year-old stem?
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What kind of cells are in the center of the stem (these will be crushed and disappear as the stem ages)
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Using higher magnification as necessary:
How do the younger vessels differ from the older vessels each year?
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What happends to the rays when they reach the phloem?
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Why should this be? ___________________________________________________________________
What are the thick-walled red stained cells in the phloem? Why do they seem to come in concentric strips? Can you find epidermis on your section and if so what is happening to it? ___________________________________________________________________
Pine stem T.S. prepared slide
What are two differences between the xylem of this plant and that in Tilia? ___________________________________________________________________
What is the difference in cell type that accounts for one of these visual differences? ___________________________________________________________________
Just as with the leaves the wood of pine is adpated to xeric conditions. Can you explain the basis of this adaptation by comparison with basswood which is a mesic species? ___________________________________________________________________
Aesculus (buckeye) twigs
What kind of leaf arrangement did this plant have?
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Does your twig represent a single stem (monopodial) or has it changed over to grow from an axillary bud at some time (sympodial)
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How old is your twig?
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How do you know?
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How many vascular bundles were present in the petioles of the leaves?
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How do you know?
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How many vascular bundles were present in the petioles of the leaves?
What other external features can you see (on the internodes) and what would be the corresponding features of herbaceous stems (If any)?
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Start peeling away the scales from the terminal bud. Examine the internal structures with the dissecting scope as you go.
What are these scales?
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After the scales are gone what are the next structures you come to, what does each feathery structure represent?
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What is in the middle of the bud?
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Take another apical bud and cut it longitudinally with a blade. This will give you another view of the structure of a bud. What are the most conspicuous features of the developing flowers?
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Wood specimens
During the lab make sure you see each of the four types:
red oak, red maple, loblolly pine and bamboo.
Is the bamboo really "wood"; can you see distinct regions of bark (periderm and phloem) and wood (or xylem)?
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What are the dots on the tranverse section end of the bamboo? ______________________
What is the ring around the outside of the stem?
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In the true "woods", what feature marks the site of the cambium.
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Where is the phloem relative to this feature?
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On which longitudinal face of the block do the rays show up most clearly?
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Which of these woods would you describe as "ring porous"?
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In which of these woods is there evidence of a succession of localized cork cambia?
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