A student cuts across the thin root of a young sunflower plant and across its thick old root…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student cuts across the thin root of a young sunflower plant and across its thick old root. The thick root shows a ring of secondary xylem round a small core of primary xylem.
- Where does the cambium of a dicot root first appear?
- How does the cambium become a round ring, when it begins as separate wavy strips?
- Why is the primary xylem found near the centre of the old root?
Need a reminder? The key ideaShow
Secondary growth makes dicot stems and roots thicker. It is done by the vascular cambium and the cork cambium. A dicot vascular bundle is open, with fascicular cambium between the xylem and the phloem. Interfascicular cambium forms from cells of the medullary rays, and the two join into a ring. The cambium cuts off secondary xylem inward and secondary phloem outward, and makes much more xylem than phloem. Monocot bundles are closed and have no cambium. In a dicot root the cambium arises from conjunctive tissue and the pericycle.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) In strips of conjunctive tissue just below the phloem. Cells of the pericycle, opposite the xylem, also start to divide.
Step 2. (b) The strips join into a wavy ring. The cambium makes more xylem than phloem, and the new xylem pushes the ring outward until it is round.
Step 3. (c) The cambium cuts secondary xylem on its inner side, outside the old xylem. So the primary xylem is left in the middle.
Answer: (a) below the phloem and in the pericycle; (b) the strips join and the added xylem rounds the ring; (c) new xylem is added outside it.
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