On the Sea-grasses in Japan (III). General Consideration on the Japanese Sea-grasses By Shigeru Niiki Received October 3, 1933 I. Characters of Sea-Grasses A. Shape B. Morphological Differences from the Allied Fresh Water Plants II. Distribution of Sea-Grasses A. General Distribution B. Sea-Grasses in the World C. On Non-Occurrence of Posidonia in Japan III. Floristic Consideration of Sea-Grasses in Japan A. Zosteraceae B. Cymodoceaceae C. Ilydrocaritaceae IV. Summary V. Literature There are eight genera of sea-grasses known in the world. Of these, with the exception of Posidonia, seven remaining genera represented by 15 species, are found in Japan. They have been described in my previous papers (9, 10 and 11). Among them the great majority of Zostera and Phyllospadix species are endemic, while Gym odocea, Diplanthera except one, and all species of the Hydrocaritaceae belong to the Indo-1\Ialay element. The morphological characters of sea-grasses together with their ecological and floristic distribution in general may now be considered. I. Characters of Sea-Grasses A. Shape The leaf is ribbon-like in form except in Halophila and the vegetative shoots grow all the year round except in the northern form of Zostera nana. The arrangement of leaves vary in different families; in the Hydro- 172 THE BOTANICAL MAGAZINE (vol. XLVIII,No. X67 caritaceae they are arranged on the upper and lower side of the rhizome and without axillant leaf on the branches except the rhizome of T halassia (11), while in all other families leaves are attached to the lateral sides of rhizome, and each branch is provided with an axillant leaf. The flower has a complete perianth only in the Ilydrocaritaceae, while inn other families there is no distinct perianth. The shape of the pollen grain is conf ervoid except in the Ilydrocaritaceae. B. Morphological Differences from the Allied Fresh Water Plants If we compare the characters of sea-grasses with those of the allied fresh water genera, following differences may be found, viz. As shown in the table the shoots of sea-grasses are more flexible against the wave motion of water than the fresh water plants, the differen- tiation of leaves according to the developmental stages or position of shoot is also meagre. These characters are in accordance with biological conditions, that in the sea there is incessant movement, nearly constant transparency and fairly constant air contents of water in contrast with the fresh water, where the stagnancy accompanied by light opacity and destitution of oxygen makes the condition quite uneven according to the depth. II. Distribution of Sea-Grasses A. General Distribution The ranges of distribution of sea-grasses are found to be very limited compared with those of fresh water plants. This may be due to the grow- ing condition, viz. 1) the summer temperature of water is lower in the sea than in the fresh water, 2) sea-grasses do not form the rest organ March20, 1934] S. MIKI-ON TEE SEA-GRASSES IN JAPAN (III). 173 capable of propagation and the seed formation is rare except in Zostera marina, Z, nana and Halophila ovalis. B. Sea-Grasses in the World Sea-grasses are usually included in two families, viz, fIydrocaritaceae and Potamogetonaceae (2, 13) but the latterr family contains three different groups of sea-grasses, each of which has been regarded by some investiga- tors as a distinct family. Thus, Zostera and Phyllospad ix are regarded to belong to the Zosteraceae because of one sided apadix , (ymodocea and Diplanth-era to the Cymodoceaceae because of the solitary flower , and Posidonia to the Posidoniaceae because of the characters of the flower and fruit (8). The distribution of sea-grasses has been discussed by many authors : ASCITERSON, who is realy the first investigator of sea-grasses, discovered the intimate relation of Indo-pacific and Calibbean Sea. I)UDL EY has sup- posed the birth place of sea-grasses to be Australia as there are found many more species compared with other regions. OSTENFELD distinguished 8 groups of sea-grasses by their characteristic species and discussed their origin. The writer is inclined to consider it more reasonable to divide the sea-grasses into 4 families as mentioned above because of their distinct characters. The distribution of these families is shown in the following table. The regions where the different families of sea-grasses predominate vary according to the families, viz. Ilydrocaritaeeae in the Indo-Malay region. Cymodoceaceae in Australia and the adjoining regions. Zosteraceae in Oriental Asia. Posidoniacea~ in Australia. The region where the representatives of a family are distributited densely and where the plants are not destitute of flowers, may be generally regarded as the original home of these forms. C. On Non-Occurrence of Posidonia in Japan Posid onia is found only in west Australia and Mediterranean Sea. On account of such isolated occurrence, as well as of fossil remains (2) the genus is supposed by OSTENFELD (12) as having originated in the tropical sea and remaining now on both sides of the equator. The fossil remain which has been designated as Posidonia is, however, only a piece of a rhizome and does not constitute a sufficient material for the determination of genus. Further-more the degeneration of the flower 174 THE BOTANICAL MAGAZINE [Vol. 1LVIII, Xo. 507 March20, 1934] S. MIKI--ON THE SEA-GRASSESIN JAPAN (III). 175 structures in this genus does not reach that degree of advancement coin- monly associated with a long life in the sea. Had it lived long in the sea, moreover, it must have shown a much wider distribution so as to include either Calibbean. Sea or Oriental Asia. III. Floristic Consideration of Sea-Grasses in Japan A. Zosteraceae a. Zostera in Japan there have been found many species of subgenera Alega, but of subgenera Zosterella there is only Zostera nana known. The latter has more primitive characters than the former group in regard to the primary root and retinacula (10). Allied species of the latter occur in Australia, viz. Zostera capricorni, Z. Muelleri and Z. tasmanica. Zostera nai a in Japan is represented by three main forms continuous in character. This species may be supposed as introduced into the Japanese waters from Australia recently, because the distribution of Zosterella in the world is nearly concentric from Australia and it is not yet found in America. b. Phyllospadix Phyllospadix has many more advanced characters than Zostera except the retinacula and has been supposed as derived from Zostera by many authors (5, 6, 8 and 13), their distribution being restricted to northern Pacific Ocean. But, as it still has a well developed retinacula, it must have been derived together with Zostera from a genus not living now. The birth place of both genera, Zostera and Phyllospadix, has been supposed to be Australia or tropics by DUDLEY(6) and OSTENFELD(13). Put, as it has been already mentioned, there are found allied fossil remains from Cretaceous (7) in Japan and also there are many endemic species in Japan (9 and 10), therefore, it seems to be more reasonable to conceive, that the family has originated in Oriental Asia rather than in the tropics or Australia. In an early geological time, possibly in the beginning of Tertiary, their habitats might have been separated to the north and south of the equator, thus making 2 centers as at present. As to the origin of American Phyllospadix, it may be pointed out that they have many roots developing on the hind side of each leaf except in the seedling which has two roots in each node. This must be con- sidered a new character because the rest of the Zosteraceae have two or two tufts of roots. Moreover, the two species in America possess charac- ters more or less common with each other indicating a rather recent differentiation, while the two species in Japan are distinctly and specifically 176 THE BOTANICAL MAGAZINE [Vol.XLVIII, No. 567 different. All these observations make it very probable that the American species have been derived from the Japanese prototype which were carried over there by the sea current. B. Cymodoceaceae The family is much simpler in the characters of flowers than the preceding family and is distributed only in the tropical region of different seas. a. Gym ,odocea The species of this genus in this country have either no flower at all or only the female flower, but those in east Africa (12) and west Australia (14) have both kinds of flowers and also there are many more species there than in other regions. The birth place may be supposed, therefore, as east Africa or west Australia, and if so the Japanese species must have been introduced from there. b. Diplanthera The distribution of this genus is wide. Judging from their position, the scales on the basal portion of the anther of the staminate flower may be taken as abortive perianthes. With this one exception the morphological characters of this genus are similar to those of Cymod ocea, so that it is not unreasonable to suppose the former as having been derived from the latter. C. Hydrocaritaceae a. Enhalus and Thalassia The range of distribution of Thalassia is wider than that of Enhalus. The genetical relation of these two genera is not clear, because their morphological characters deviate far apart. The original home of Thalassia is difficult to ascertain since the two known species are nearly alike in their mode of differentiation. But so far as can be decided from the range of distribution, the genus seems to have originated in the Indo- Pacific region, because T.