The amount of callose observed varies with season, age, metabolism. Callose synthase is in the cell membrane. These cells have a dense cytoplasm, mitochondria, nucleus, golgi, ER, chloroplasts - the standard goodies. These cells are derived from the same cambial initial cell as the sieve tube members. These are vacuolated, storage cells.
Transfer cells are specialized parenchyma cells.
Fibers - primarily for support. Appear early in development of sieve elements. Only in angiosperms at least two proteins, PP1 and PP2 Once the sieve pores form, the p protein disperses through the pore. The protein is fibrillar P protein plugs the pore when the cell is damaged. Mechanism for phloem transport Requirements.