The xylem is made from dead, hollow cells.
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Tracheids and vessel elements are the main components of xylem witch have a function to transport water. Those are elongated dead cells at maturity (without protoplast) with a thick, strengthened cellulose cell wall and a hollow lumen. Even though they are dead they still maintain a function, because are being protected by surrounding living cells.</span>
Answer: (B) Pre-zygotic; post-zygotic.
Explanation:
Prezygotic isolation occurs before fertilization of eggs. It occurs between the two sex gametes and deter sexual reproduction between different species.
However, individuals are considered to be diverge on the phylogeny tree and different species, when they cannot reproduce.
Mechanisms of prezygotic isolation include habitat isolation, gamete isolation, mechanical isolation, behavioral isolation, and mating seasons.
Whereas, postzygotic isolation prevents the formation of fertile offspring. This phenomenon leads to production of an hybrid offspring from different species and this is a form of speciation in nature.
Mechanisms of postzygotic isolation include hybrid inviability, hybrid breakdown, and hybrid sterility.