The most common and most likely answer to this crossword clue is the 9 letter word DENDRITES
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The straight part
We know that "crystalline structures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crystalline structures = dendrites.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DENDRITES mean?
DENDRITES noun- A slender projection of a nerve cell which conducts nerve impulses from a synapse to the body of the cell; a dendron.
- Slender cell process emanating from the cell bodies of dendritic cells and follicular dendritic cells of the immune system.
- Tree-like structure of crystals growing as material crystallizes
- A hermit who lived in a tree