Chap. II] PERIODIC PHENOMENA IN TEMPERATE ZONES 441 



follows only after frost, and is confined to leaves exposed to the direct 

 rays of the sun. The yellow colour is due to a partial decomposition 

 of the chlorophyll, the red colour is caused either by a red pigment in 

 the chlorophyll-corpuscles (Thuya, Buxus), or by anthocyan in the cell-sap 

 (Hex, Hedera, Mahonia) 1 . Many species of Pinus assume a characteristic 

 appearance in winter because their fascicled needles become adpressed 

 to the twigs, owing to certain hitherto unexplained physiological causes. 



In contrast with woody vegetation, evergreen species predominate in 

 the herbaceous vegetation, so that meadows retain their fresh green 

 appearance in mild winters and present yellow tints only during persistently 

 hard frost. Many herbs however develop anthocyan during the winter, 

 but as this discoloration is usually absent from grasses, it is far less 

 apparent than among woody plants. Many herbaceous perennials are 

 only transitorily green ; the death of the subaerial shoots frequently 

 happens even during summer, and is therefore probably due to inherent 

 causes. 



It is well known that flowers are never entirely lacking at any season, 

 as during mild weather many species, like Bellis perennis, Senecio vulgaris, 

 Veronica hederaefolia, blossom even in winter. True winter-flowering 

 plants, however, do not occur in the markedly cold belts of the temperate 

 zones ; the species just mentioned flower really throughout the year 

 and still more freely in the warmer seasons. On the other hand, the 

 warmer belts produce a number of plants whose flowering season falls 

 in the months from November to February. Rein 2 makes the following 

 remarks regarding the winter-flora of Japan: — 



' Towards the end of October the woodland that is green in summer 

 is leafless, as with us [i. e. in Central Europe], and there are only a few 

 plants that have not entered upon their winter rest. These are chiefly 

 shrubs and trees that are green in winter . . . the flowering of which 

 occurs during the earliest winter months. Among them are Olea 

 Aquifolium, Sieb. et Zucc, Aralia japonica, Thunbg., and some other 

 Araliaceae, which blossom in November ; Thea chinensis, Sims, and 

 Camellia Sasanqua, Thunbg., whose flowering time is in November and 

 December, and whose last buds are killed by night-frosts ; a few species 

 of Daphne, which blossom in January and February ; and above all 

 Camellia japonica, which in this season sometimes affords the surprising 

 spectacle of bearing simultaneously flowers and snow, but whose flowering 

 time is prolonged until April. 



'Among herbs we find still fewer species whose flowering time falls 

 in late autumn or actually extends into winter proper ; among such are 

 a few Compositae, especially Pyrethrum and Aster.' 



Even in the warmer belts winter-flowering plants are scanty. The 

 1 Schimper, op. cit., p. 166. 2 Rein, op. cit., p. 155. 



