212 PERIODICITY OF GROWTH 



The optimum growth is frequently only attained under a definite succession 

 of changing conditions, and in the raising of early crops, for example, the best 

 results are obtained when the temperature gradually rises as development pro- 

 gresses '. Those plants which have a low maximum temperature must enter into 

 a resting condition during summer. It is probable, but not yet certain, that the 

 summer resting period of Ranunculus Ficaria is the result of an autogenic 

 periodicity, while that of Ulothrix zonata and Hydrurus is due to the direct 

 action of the high temperature. 



Mean temperatures. Even when all other factors are constant, the relationship 

 between growth and temperature cannot be expressed in the form of a simple 

 equation (by a straight line), and this is still less the case when the plant's activity 

 and power of response are influenced by simultaneous changes in other conditions, 

 such as always occur in nature. Hence there is no constant relation between the 

 mean temperature and the time of development, although a general correspondence 

 will be shown when the climate maintains the same annual rhythm. A long period 

 of drought would, however, by retarding vegetation temporarily alter the mean 

 temperature for growth, which also differs for the same plant under dissimilar 

 climatic conditions. Another instance of variation is shown by the fact that many 

 of our flowers do not always bloom in the same order 2 . 



SECTION 61. The Origin of Yearly Periodicity. 



The first question to determine is whether the autogenic resting period 

 is a fixed hereditary property, or whether it is merely an induced character- 

 istic which can be shortened or removed under constant external conditions. 

 That it is capable of a certain modification has already been seen, for not 

 only may it be shortened by previous stimulation, but cultural varieties are 

 readily produced in which the resting period differs in length from that of 

 the parent stock. Volkens in fact states that in potato-tubers grown at 

 Kilimanjaro the resting period has disappeared, and probably the same 

 may be found to occur in numerous lower organisms under appropriate 

 conditions. 



The autogenic resting period seems also to shorten or disappear when 

 deciduous trees are grown under as uniformly favourable conditions as 

 possible. Thus at Tjibodas (5,000 feet) on the flanks of the volcano of 

 Gedeh, where the climate is temperate and constantly humid, Qncrcus 

 pediinculata, Pyrns Mains and P. communis, Liriodcndron tnlipifera, 

 Amygdalus covinntnis, and other deciduous trees grow like evergreens. The 

 buds expand not simultaneously but successively, so that an individual plant 



1 Cf. Schimper, Pflanzengeographie, 1898, p. 469 ; Kerner, Pflanzenleben, 1887, i. Aufl., Bd. i, 

 p. 525; Miiller-Thurgan, Landw. Jahrb., 1885, Bd. XIV, p. 903. 



2 See Brack, Handb. der Pflanzengeographie, 1890, p. 39; Grisebach, Vegetation d. Erde, 1872, 

 Bd. i, p. 227 ; Koppen, \Varme u. Pflanzenwachsthum, 1870, p. 54; Sachs, Jahrb. f. wiss. Bot., 1860, 

 Bd. ii, p. 370. 



