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Please correct me if my understanding is incorrect; I understand branch/stem, and root are composed of different types of cells. Yet in some plants (e.g. rose, bougainvillea) a cutting from the stem is capable of developing and taking root.

How does this transformation from stem to root come about?

As an aside - I believe such root-generation happens only in plant cuttings, and not in a tree branch. Why? Is it a matter of the volume of nutrition required?

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Plants grow only from regions at the tips of the roots and shoots called meristems.

Within the meristem areas there are stem cells ("blank" unspecialised cells). Unlike animal stem cells, plant stem cells are totipotent - meaning that they can differentiate into any type of cell. Therefore when the cutting is taken from the end of the shoot, the stem cells can differentiate into root cells or shoot cells depending on their conditions.

Because the meristems (therefore the unspecialised stem cells) are only located at the tips of shoots, you cant grow a cutting from the middle of a branch.

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There are lateral meristems all along a plant - they are where branches grow from. Every node on a plant is, or was once, a meristem. That's why you cut just below a node to take a cutting. You could also create a meristem pretty much anywhere by applying auxin. That's why people use rooting/cutting powder, which is NAA, a synthetic auxin. –  Richard Smith Jul 8 '12 at 20:02
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