Deciding Monocot or Dicot from Stem Sections and a Leaf
Two permanent slides are placed in front of a first-year student. Slide A shows a stem with 16 vascular bundles arranged in a ring. Each bundle has xylem and phloem on the same radius, cambium between them, and the smallest xylem vessels towards the centre (endarch). The cortex is narrow, the hypodermis is collenchymatous and the pith is large. Slide B shows a stem with many vascular bundles scattered through the ground tissue. Each bundle is surrounded by a sclerenchymatous bundle sheath and has no cambium; xylem and phloem lie on the same radius. The student calls B a young herbaceous dicot because the plant is thin, and predicts that after three years a transverse section will show three annual rings. A third slide, C, is a root with radial bundles and exarch xylem. The teacher asks the student to re-examine the prediction and to use leaf material from plant B, which shows parallel venation and a leaf sheath, as a check.
1. Identify slides A and B, and give two anatomical criteria that decide each identification.
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Slide A is a dicot stem. First, the vascular bundles are arranged in a ring, not scattered. Second, each bundle is open because cambium lies between xylem and phloem, and the xylem is endarch, with the smallest vessels towards the centre. The large pith and collenchymatous hypodermis are consistent with a dicot stem.
Slide B is a monocot stem. First, the vascular bundles are scattered through the ground tissue rather than arranged in a ring. Second, each bundle is closed and surrounded by a sclerenchymatous bundle sheath, with no cambium. The absence of a cambium is the key functional difference, because it removes the tissue that produces secondary xylem and phloem.
2. Evaluate the prediction that plant B will show three annual rings after three years. Describe what a section of plant A would show after the same period.
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The prediction for B is wrong. Annual rings are secondary xylem produced by a cambium. In B the bundles are closed and lack cambium, and there is no continuous cambial ring between the bundles. After three years, a section of B would still show only primary tissues: epidermis, ground tissue, scattered closed bundles and bundle sheaths. It would not show three rings.
In A the fascicular cambium is already present inside the bundles. In a growing stem it joins with interfascicular cambium to form a complete cambial ring. This ring produces secondary xylem internally and secondary phloem externally. After three years, a section of A would show three growth rings of secondary xylem, with the oldest towards the centre, together with medullary rays and compressed primary tissues towards the outside. The presence of an open bundle is therefore the anatomical reason for the different prediction.
3. Explain how the leaf material from B corroborates the anatomical conclusion, and state why slide C cannot be used to decide the stem type.
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The leaf from B has parallel venation and a leaf sheath. Both are characteristic monocot leaf features and agree with the scattered, closed vascular bundles seen in the stem. The two independent sets of evidence, leaf and stem, point to the same group, so the identification is strengthened.
Slide C is a root, not a stem. Its vascular bundles are radial, with xylem and phloem on different radii, and the xylem is exarch, with the smallest vessels towards the outside. Those are root characters. A root section can help decide monocot or dicot only if additional features such as the number of xylem poles are known; radial and exarch bundles alone do not settle the stem type. Using C to classify the stem would mix organs with different ground plans.