藻苔纲Takakiopsida，一个独特的苔藓植物类群

藻苔属Takakia在建立之初被认为是原始的苔类植物。这一神秘类群的出现引起了苔藓学家的兴趣,发表了一系列相关的论文。但是随着其孢子体的发现,它的系统位置引起了争论。本文详细分析了藻苔属植物的形态性状特征,参考了大量其他学科的研究成果,认为它既不能归于类,也不能归于苔类。就其系统位置而言,应该成为一独立的纲——藻苔纲。

Takakiopsida, a unique taxon of bryophytes

The genus Takakia was established by Hattori and Inoue in 1958 when the gametophyte of T. lepidozioides was found in Japan. In 1963, a species described by Mitten in the hepaticae genus Lepidozia, L. ceratophylla, was transferred to this genus by Grolle. Since its establishment, this enigmatic genus of two species has been the subject of intense scrutiny and phylogenetic debate, and many papers dealing with its taxonomy, morphology and phytogeography have been published. In this paper we discuss the morphological characters of this genus in detail and analyze much information from spore morphology, cytology, phytochemistry, molecular systematics, phytogeography and paleobotany in order to have a better understanding on its systematic position. Gross_morphologically the sporophytes of Takakia are similar to those of some moss groups such as Andreaea and Andreaeobryum, but the gametophytes are very similar to those of some liverworts, e.g. Haplomitrium and Blepharostoma. Evidence from sporophyte development and antheridial ontogeny indicates that Takakia is neither a moss nor a liverwort. Cytological data indicate its two species is not of very close relationship. Phytochemical data, although very limited, suggest a closer affinity of Takakia to hepaticae than to mosses. There is no exact counterparts in gross sporophyte morphology with Takakia among extant archegoniates, although striking parallels are evident in ancient fossil land plants. The shape and spiralled external architecture of the stem and sporangium of Tortilicaulis , a lower Devonian plant, are remarkably similar to that of Takakia . Takakia as a primitive genus is undoubtedly a unique type, which is systematically isolated in the bryophytes, and has both characters of mosses and liverworts. From these facts we consider that Takakia might have contained more than two species in history, but only two of them have survived. In the long historical geological period, two diverse types, one including liverworts and the other including mosses, might be derived from Takakia, and the intermediate ones between the two types might have disappeared. Those genera which are similar to Takakia in sporophyte morphology, such as Andreaea and Andreaeobryum, or in gametophyte morphology, such as Haplomitrium and Blepharostoma, are commonly mono_ or stenotypic, each including few or only one species, and are exceptionally isolated phylogenetically and thus are relict elements. All of them, including Takakia, therefore, may represent the remnants of the earliest bryophytes. The results of molecular systematics have shown that liverworts, hornworts and mosses are derived from a same ancient bryophyte in different periods of time, of which the liverworts and hornworts diverged earlier, while mosses diverged later. All available evidence suggests that Takakia is a "basal" group in bryophytes. Considering its isolated systematic position we agree with Schuster to treat it as an independent class, Takakiopsida.