<国立科博専報>陸中における Nilssonia の進化と気候の変化

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Plants change their forms according to changes in their environment, and there are two ways of transformation. One is the case where the environment changes favorable and accelerate the plant growth (GA : Growth Acceleration), thus causing a change in the form of the plant. The other case is where the environment changes so as to retard the plant growth (GR : Growth Retardation), which also results in a plant form change. The short-period changes of plants in geologic age and the longperiod changes from the Paleozoic to Cenozoic are, for the most part, due to GR. As the cause of the Growth Retardation, the writer considered that the climate had changed in a definite direction that the annual difference of temperature widened. Reduction of the surface area of leaves is a remarkable feature of the change of plant form. This is comparable with the increase in the size of animals. Both plants and animals are changing their forms according to the change (toward deterioration) of their environment. The writer collected numerous specimens of Nilssonia in the Rikuchu region, Northeast Japan, and compared the width of leaves. The specimens were collected from three localities, Hosoura (Lower Jurassic), Oshima (Upper Jurassic) and Omoto (Lower Cretaceous). As shown in Fig. 1,the frequency distribution of the leaf-width of the specimens from the three localities obviously shows three peaks separated one another. Specimens from Hosoura and Oshima were reported previously (OISHI, 1932) as N. orientalis and those from Omoto as N. schaumburgensis var. parvula. It is evident that N. orientalis form Oshima should be distinguished from the larger type from Hosoura, and is referred to N. orientalis var. minor FONT. (in WARD, 1905) Specimens from Hosoura (Lower Jurassic) are the largest in size, those from Oshima (Upper Jurassic) are medium, and those from Omoto (Lower Cretaceous) are the smallest. Therefore, it is clear that the leaf size of N. orientalis decreased with the lapse of time. When the specimens hitherto reported as N. orientalis in Japan are examined, as indicated in Fig. 1,four measurements of the specimens from Iwamuro (Lias) fall within the leaf-width range of Hosoura, and another is a slightly smaller; those from Nariwa (Noric) correspond to the large type of Hosoura; and those from Tetori (Upper Jurassic) are all included in the width range of Oshima. Thus, the evolution of N. orientalis in the Rikuchu region can serve as a standard in Japan.
1968-03-30

<国立科博専報>陸中における Nilssonia の進化と気候の変化

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