Chap. II] PERIODIC PHENOMENA IN TEMPERATE ZONES 439 



the normal period of growth. For the plum alone temperatures some- 

 what higher than those prevailing In spring and summer in Central Europe 

 are favourable for the development of flower and fruit, so that, in contrast 

 to other fruit-trees, the period from the opening of the buds to the ripening 

 of the fruit can be considerably shortened. Pynaert's table of temperatures 

 suitable for forcing the peach-tree has already been given l , and is worthy 

 of study on account of its importance and comprehensiveness. 



What is true of the fruit-trees in Central Europe is also certainly true 

 of the great majority of the woody plants, as of the whole indigenous 

 flora. In other words, the flora of the temperate zones during the vegetative 

 season enjoys an occological optimum temperature, so that all the functions 

 work harmoniously side by side. It accordingly follows, as has been 

 already explained, that a transference to a climate with a higher or lower 

 temperature causes discordant changes, at any rate to begin with. Subse- 

 quently, in certain species, acclimatization is secured by adaptation to the 

 new conditions of temperature 2 , provided these conditions do not differ 

 too greatly from those of the native home. 



iv. PERIODICITY OF HERBACEOUS PLANTS. 



That the facts already established in reference to woody plants are also 

 true of perennial herbs is proved by all our experience, which has taught 

 us in particular that in perennial herbs, as in woody plants, there is a 

 resting period due to inherent causes, and that a series of metabolic changes 

 also take place in them, like those which proceed in woody plants. 



Miiller-Thurgau has thrown much light on the periodicity of the potato. The 

 well-known sweet taste of frozen potatoes is not, as is generally supposed, a 

 consequence of the frost, but sets in at temperatures between o° and +6° C. as 

 a consequence of the conversion of part of the starch into sugar (glucose, invertose). 

 Miiller-Thurgau is of opinion that we may possibly infer from this phenomenon 

 that in the potato there is a continual process of solution and of formation of 

 starch, and that the formation demands higher temperatures than the solution, 

 so that when near freezing-point there is an accumulation of sugar which at 

 a higher temperature would be converted into starch. Against this view the 

 fact may be urged that the production of sugar is essentially associated with 

 the winter ; in September and October there is no production of sugar, or the 

 production is carried on to a much less extent than it is later in the year. 



We have evidently to deal with phenomena quite similar to those in woody 

 plants. In this case too, as a consequence of inherent causes, the winter and 

 the summer conditions periodically alternate with each other in the protoplasm. 

 During the winter condition low temperatures between o° and +6° C. cause a 

 considerable transformation of starch into sugar, whereas during the summer 

 condition they only exert a weak and inhibitory action. Just as in the case of 



1 See p. 45. 2 On acclimatization, see p. 49. 



