INFLUENCE OF EXTERNAL CONDITIONS UPON PERIODICITY 211 



amount for such regions in the course of from two to four years. It also 

 takes some time for seeds developed in the South to acquire the periodicity 

 peculiar to the North 1 . 



Cieslar 2 observed that seedlings of the pine and larch produced by seeds 

 grown in a cool locality developed less rapidly than those from seeds grown 

 in warmer regions. This may be the effect of an induced after-effect, 

 although further research is required in this direction. Kienitz 3 in fact 

 finds the contrary to be the case with some seeds. 



It is not yet certain whether the shorter time of ripening of grain in the 

 North is due to the longer daily illumination or to other factors, and the same 

 applies to such plants as Gentiana campestris, Parnassia palustris, and Calluna 

 vulgaris, which flower earlier in alpine localities than in the valleys beneath 4 . 

 This result can hardly be due to the slight differences in the amount of illumina- 

 tion, and it has not even been ascertained whether the conditions for the shorter 

 summer period are induced during the long winter resting period, or whether the 

 shortening of the summer period is due to the direct action of the external 

 conditions at this time. In some cases correlative actions come into play, as 

 when a retardation of the vegetative activity hastens the formation of reproductive 

 organs. [In a deep valley whose sides slope at an angle of 30 light is received from 

 an arc of 120 instead of from one of 180 as on the neighbouring summits. 

 Further, at a height of 6,000 feet the oxygen-pressure is about one-sixth less than at 

 sea level, whereas under similar conditions of temperature, transpiration will tend 

 to be more active owing to the lowering of the vapour-pressure. Again the per- 

 centage of carbon dioxide is usually higher on lofty mountains, averaging -06 per 

 cent, at Chamounix and Mont Blanc instead of -03 per cent, as in the valleys 

 beneath. Lastly, air is imperfectly transparent, especially when laden with water- 

 vapour, and if actual clouds are interposed the loss of illumination is enormously 

 increased. In such a tropical country as Java the average height of the clouds is 

 about 5,000 feet, at 10 or 12,000 feet the sky is usually clear, and the total daily 

 illumination is very greatly increased on this account alone. In European countries 

 the cloud-distribution is less regular, but the average height is much lower, and a 

 lofty alpine summit will never have more cloud above it and will usually have less 

 than the valley beneath. All these factors (illumination, transpiration, percentage 

 of oxygen and of carbon dioxide) affect growth both directly and indirectly. ED.] 



1 Linsser, Unters. ii. die period. Erscheinungen der Pflanzen, Mem. d. 1'Acad. d. St.-Petersbourg, 

 1867, 7 ser., T. XI, and 1869, 7 e sen, T. xin; Schiibler, Die Pflanzenwelt Norwegens, 1873-5 ; 

 Bot. Centralbl., 1886, Bd. xxviil, p. 205 ; Nobbe, Samenkunde, 1876, p. 238 ; Wittmack, Landvv. 

 Jahrb., 1876, Bd. V, p. 613 ; 1877, Kd - VI > P- 999 5 Schimper, Pflanzengeographie, 1898, p. 55. 



3 Cieslar, Bot. Jahresb., 1895, p. 32. 



3 Kienitz, Bot. Unters. von N. J. C. Miiller, 1879, Bd - " P- Ir - 



4 Cf. Sendtner, Flora, 1851, p. 256. An after-effect may apparently be shown after a change of 

 locality. A change of climate by transposing the annual period may hasten or delay flowering 

 without shortening the time of development. Similarly plants which have been caused to flower 

 early in one year may show a tendency to do the same in the following year. Cf. Bouche, Bot. Ztg., 

 1873, p. 613. 



