The influences of day length, air temperature and solar radiation, and their interactions on the factors concerned with dry matter production were compared among four leading temperate grasses, orchardgrass (cv. Frode), perennial ryegrass (cv. Endura), timothy (cv. Senpoku) and tall fescue (cv. K-31). Furthermore, the varietal differences in the response to the day length were elucidated on timothy which showed the most sensitive response to the climatic conditions of the above four species. The materials used were thirteen timothy varieties which were developed at the different latitudinal regions ranging from 35°N to 60°N. The results obtained were as follows ; 1. At first, we discussed the differences in the response of dry matter production to the growth conditions (Table 1) which were obtained by variously combinating three climatic factors such as air temperature, day length and solar radiation. This response was called "Generaral Clmiatic Response (GCR)" in this paper, which was not the response to the individual climatic factor but was indicated as the integration of responses to the changes in three climatic factors. GCR in each factor in dry matter production was compared among four species by use of the regression analysis method originated from FINLAY and WILKINSON (Fig.1). GCR of Top/Root ratio was remarkably stronger in timothy and perennial ryegrass, and the order of the response was Ti (timothe)=Pr (perennial ryegrass)>Or (orchardgrass)=Tf (tall fescue). The orders of intensity of GCR in the other factors in dry matter production were as follows ; Ti=Or>Pr=Tf in the plant height, Pr>Ti=Or=Tf in the tiller number, Ti>Pr=Or=Tf in the specific leaf area and Ti=Pr>Or>Tf in the photosynthesis rate of single leaf. GCR of dry matter weight showed almost similar tendency in four species. GCR was shown stronger both in timothy and perennial ryegrass. It was worth of notice that each of the both species which is regarded as suitable for the quite different climatic conditions showed similarly the strong responses (GCR) to the changes of climatic factors. 2. The influences of the individual climatic factor "Specific Effect" (SE) and of the inter- action of three climatic factors "Interactive Effect" (IE) on dry matter production were analysed (Fig.2). SE of day length, IE of day length and the other climatic factors (air temperature and solar radiation) on dry matter production were largest in timothy, while least in tall fescue (Fig.2 A). And among the factors in dry matter production these effect (SE and IE) were shown most intensively on the tiller number through four species. The differences in SE of solar radiation and also IE of solar radiation and the other climatic factors scarcely existed among four species (Fig.2 B). SE of air temperature, and IE of air temperature and the other climatic factors were a little outstan ding in timothy (Fig.2 C). Judging from the climatic response features (SE and IE), as was shown in Fig.2, it became clear that there was a considerable difference in the climatic responses between timothy and perennial ryegrass both of which showed stronger GCR. That is, GCR in timothy was influenced mainly by day length, while GCR in perennial ryegrass was influenced equally by three climatic factors. 3. From the view point of comparing the features of climatic responses in four species (Fig.1 and 2), it was ascertained that timothy was desirable for cultivation under the high latitudinal regions with severe winter condition and that the production efficiency of perennial ryegrass became higher under the mild climatic condition. Tall fescue was supposed to be adapted for a wide range of latitude, since this species was scarcely influenced by the changes of day length. Orchardgrass was regarded as a species suitable for the regions with the intermediate climatic condition between the favorable clim

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