Why do phloem and xylem not contain starch?
The phloem transports sucrose that is converted to starch elsewhere. The xylem transports mostly water.
Looking at it from a different "why":
Starches are not very soluble in water, so dissolving and transporting them in water is not likely. Sugars like glucose and sucrose are soluble.
Actually this a great question. There are species that have starches in underground stems and some starch in the leaves. A priori, you would expect starch to be moved around in phloem in such a plant, at least, to account for getting the starch buildup in the underground stem, since starch is a storage product for photosynthetic sugars.
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