^4o ZONES AND REGIONS [Pt. Ill, Sect. II 



woodi' plants, at a high temperature starch is regenerated at the expense of 

 products, sugar in this instance, into which it was at an earlier stage converted. 

 The production of sugar during winter is not necessary for the further development 

 of the potato, but has an accelerating effect on it. It must be due to other causes 

 that potato-buds do not develop further in autumn. Sachs has put forward the 

 attractive hypothesis, that in this and other similar cases it may be a question of 

 the gradual formation of ferments ; an experimental proof of this suggestion has 

 not yet been attempted '. 



v. COLD AND DROUGHT. 



In their action on vegetation cold periods display an ninnistakablc likeness 

 to dry periods. That this likeness is not specious, but is founded on the 

 organization of the plant, appears from the circumstance that both factors 

 frequently influence periodicity in a quite similar manner and can replace 

 one another. Thus forcing is accelerated if water is withheld for some 

 time before the commencement of winter cold ; the winter period of rest 

 then commences and terminates sooner^. Persistent drought hastens the 

 defoliation of our deciduous trees. The buds of woody plants and 

 herbaceous perennials are no more induced to open by moisture during 

 the dry season than are winter buds by higher temperatures, so long 

 as a certain time determined by inherent causes has not been reached. 

 Plants richly provided with reserve-material blossom in the tropics chiefly 

 during the dry season and immediately after it, but in temperate zones 

 chiefly in the spring. Many trees that blossom normally in the cold 

 season have after a dry summer a second weaker flowering. Closer 

 investigation regarding metabolism during the period of vegetative rest 

 that is due to drought will show how far analogies between metabolic 

 changes and movements of reserve-material correspond to these external 

 analogies. 



2. PERIODIC ASPECTS OF VEGETATION. 



i. GENERAL CONSIDERATIONS. 



The variety in the aspect of vegetation that changes with each season 

 is mainlj' due to periodic phenomena. The most conspicuous changes 

 in the vegetative organs are exhibited by woody plants at the autumnal 

 leaf-fall. But among evergreen plants also there is in many cases a not 

 unessential difference between the appearance in winter and in summer, 

 as many Coniferae assume a brownish-yellow colour, others, as well as 

 some broad-leaved species, a brownish-red one. Such a change of colour 



^ See also Lidforss, op. cit. 



- Muller-Thurgau, II, p. 901; Pynaen, op. cit., p. 263. .According to Pynaert, the 

 result is not quite certain. 



