There are two species in
Ishige, I. Okamurai YENDO and
I. foliacea OKAM. The zoospores of the former were observed by Mrr. ONDA & HIGASHI and the latter ones by Drs. MIYAKE & KUNIEDA. However, no further development of the zoospores was reported.
Since 1936
I. foliacea OKAM. which grows from winter to summer in the vicinity of Izumi-mura, Atumi-gun, Aiti-ken, has been used for my observations. The liberation of the zoospores continues from April to August, while the best conditions are found in about the end of May.
The zoospore is elongated pear-shaped (4, 8-6, 5μ×3, 3-5, 0μ), having two cilia attached to the lateral part of the body and a chromatophore with an eye-spot. The anterior cilium is about two times as long as the posterior one. After swimming about for a few hours it comes to rest, losing its cilia and becomes round (3, 3-5, 8μ, mean 4, 6μ in diameter). It germinates and develops into a streblonemoid protonema with many branches. In summer, it ceases the further development and most of the cells in the protonema are transformed into the spherical resting cells with a thick membrane and a few oil-like globules.
In late autumn, however, each of the resting cells begins to germinate and forms a filaments with some branchlets. The front portion of the filament and each branchlet turns into the plurilocular sporangium. There are two kinds of filaments which have a differentiated sexuality, but no differences are found in shape and size of the sterile part. A slight difference is observed in size of the gametes according to sex. The macrogamete is about 4, 7-6, 0μ×2, 7-4, 5μ when round 4, 3μ in diam. in size, while the micro-gamete measures 4, 0-5, 2μ×2, 5-3, 6μ (3, 4μ in diam.). Both gametes are pear-shaped, having two cilia attached laterally and a chromatophore with an eye-spot. When the macro- and micro-gametes meet they conjugate with each other.
The zygote germinates, developing into a creeping protonema. A few filaments give rise from the protonema and grow upward, branching out many ramification. The upright filaments become entangled with one another forming an“Anlage”of the main axis of the young
Ishige-plant (sporophyte). Though its further development beyond this has not been followed, it is very probable that the germinating zygote produces an
Ishige plant.
Every non-conjugated gamete is also able to germinate parthenogenetically, and develops into a microscopical gametophyte on which the gametangia (plurilocular sporangia) are again formed.
Therefore, from these observations, the life-history of the
Ishige foliacea is illustrated as follows: the plant collected in the sea represents the asexual generation while the sexual generation is microscopic, from this two-ciliated, motile, macro- and micro-gametes are liberated.
From the standpoints of the life-history and the development, “
Ishige” seems to have some nearer relation with Chordariales.
View full abstract