arising peripherally from parts of the root or stem and having no connection to meristems

Merriam-Webster Online Dictionary
adventitious (adjective)
1.
coming from another source and not inherent or innate - a Federal house without adventitious later additions
2.
arising or occurring sporadically or in other than the usual location - adventitious roots
adventitious (Wikipedia)

Plants produce new tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal embryo will very early produce all of the body parts that it will ever have in its life. When the animal is born (or hatches from its egg), it has all its body parts and from that point will only grow larger and more mature.

The properties of organization seen in a plant are emergent properties which are more than the sum of the individual parts. "The assembly of these tissues and functions into an integrated multicellular organism yields not only the characteristics of the separate parts and processes but also quite a new set of characteristics which would not have been predictable on the basis of examination of the separate parts."

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