The balloon-shaped structures called tyloses:

Options

A. Are extensions of xylem parenchyma cells into vessels
B. Are linked to the ascent of sap through xylem vessels
C. Originate in the lumen of vessels
D. Characterise the sapwood

Answer

**Step-by-Step Solution:**

1. **Understanding Tyloses**: Tyloses are balloon-shaped structures that form in the xylem of certain plants, particularly under unfavorable conditions. They are extensions of xylem parenchyma cells that protrude into the xylem vessels.

2. **Function of Tyloses**: The primary function of tyloses is to store food and potentially block the flow of water in the xylem vessels. This can be a response to stress conditions, such as drought or injury, where the plant needs to conserve resources.

3. **Analyzing the Options**:
- **Option A**: "Extension of xylem parenchyma cells into vessels." This is correct, as tyloses indeed originate from xylem parenchyma cells and extend into the vessels.
- **Option B**: "A link to the ascent of sap through xylem vessel." This is incorrect because tyloses do not facilitate the ascent of sap; instead, they can obstruct it.
- **Option C**: "They originate in the lumen of vessel." This is incorrect as tyloses do not originate from the lumen of the vessel; they extend from parenchyma cells.
- **Option D**: "Characterize the sapwood." This is also incorrect since sapwood refers to the functional xylem that conducts water and does not directly relate to the presence of tyloses.

4. **Conclusion**: Based on the analysis, the correct answer is that tyloses are an extension of xylem parenchyma cells into vessels.

**Final Answer**: Tyloses are the extension of xylem parenchyma cells into vessels.

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