Enucleate condition means cells without nuclei. This is found in vessel elements and sieve tube elements. Thick secondary wall is the inner most layer of cell wall, it is found in the cell wall of vessel elements. The thickened areas present on the lateral walls of the vessel elements are called pores. P- proteins are the small tubular fibrillar, granular or crystalline sub microscopic structures appear in the cytoplasm of sieve tubes.
Caution
Xylem vessels become dead and lose their protoplasm due to deposition of lignified secondary wall mature sieve tube elements are living cells without nucleus.