Many morphological studies on sieve tubes of Phaeophyta have been reported on many
species of Laminaria，Macrocystis，Nereocystis，Fucus，etc.　In the present study，several coincidences as well as differences were found in the fine
structure of sieve tubes of Undaria pinnatifida as compared to those of other algae reported as above. Lateral cell wall of sieve tube cells of U. pinnatifida was made of a bilayer structure like other species of Phaeophyta. The sieve tube cells contained organells，such as nuclei，
chloroplasts，mitochondria and vesicular cytoplasm. The organelles were similar to those found in Laminaria，Nereocystis and Pelagophycus. The sieve plate of U. pinnatifida was composed of the same kind of material as that of lateral wall of the sieve tube cell. In comparison of number， density and diameter of the sieve pores with those of Pelagophycus，Macrocystis，some difference was detected even among the Pheophyta，i. e.，one sieve plate had many small sieve pores and another one
possessed not many，but big sieve pores.
Oncogenesis of sieve tubes，sieve plates and sieve pores of U. pinnatifida could be
observed in a specimen whose leaf was 3-5 cm in length. It is suggested that the sieve
tube is formed gradually after cell division of medulary layer and subsequent removal of the septum between the two cells. Callus，known to be formed in sieve plates of higher plants，Laminaria and Macrocystis，
was not detected in U. pinnatifida. The sieve tubes of U. pinnatifida lacked companion cells. Absence of companion cells has also been confirmed in Laminaria and Macrocystis. The lacking may be a major difference from sieve tube tissues of higher plants.