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"Assistant" does not make sense in your first sentence. An assistant is a person who ranks below a senior person -- "the managing director and her assistant".-- or who helps in particular work: "a laboratory assistant". Do you mean "arrangement"?

In your first sentence, "bundle" should be plural -- bundles. "Where is" forms a question, but the rest of your sentence does not seem to be asking a question. Perhaps you mean "where".

"Gave" should be present tense like the rest of your sentence -- give. "Stable" in your sentence is an adjective, which should not modify a verb. You want the noun form - stability.

Your sentence ends with a nonrestrictive relative clause beginning with "which".  The "which" should be preceded by a comma. However, because this clause cannot be removed without altering the basic meaning, it should be a restrictive clause beginning with "that". No comma is required.

"in mechanical properties palm trunk" does not make sense. Perhaps you mean "give mechanical stability to the palm trunk."

"This resulted from a different arrangement of parenchyma cells and vascular bundles, where the peripheral zone is composed of a small number of parenchyma cells and a large number of vascular bundles that give mechanical stability to the palm trunk."

Sanjay asked me to comment on preposition sprawl -- the tendency to overuse prepositional phrases in a sentence. More often than not, a prepositional adjective phrase can be replaced by a direct adjective. For instance, "arrangement of parenchyma cells and vascular bundles" can be more concisely stated as "parenchyma cell and vascular bundle arrangement."

The phrase " of a small number of parenchyma cells and a large number of vascular bundles" includes three prepositional phrases, which is too many by most standards. But I don't see a good alternative.

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Jeff, Sir keep writing  on the forum to avoid prepositional prawls for the better writing. Your sentence contains only one period at the end. Please wait for the other experts to jump in to answer your question.