THE ORIGIN OF YEARLY PERIODICITY 215 



incorrect in assuming that the resting period is the direct result of a deficiency of 

 food capable of use l . Sachs 2 suggested that the cessation of the resting period was 

 due to the gradual production of solvent enzymes rendering food-materials available, 

 while Miiller-Thurgau supposed that the sprouting of a potato-tuber was induced in 

 it owing to the conversion of some of the starch into sugar. The commencement 

 of growth naturally involves an increased metabolism, but the latter may also occur 

 as the result of injury without the plant being awakened from its resting period. 

 It is in fact not at present possible to say what part is played in the production 

 of periodicity by stimulating or nutrient substances. 



Annual rings. Nothing definite is known as to the causes which 

 determine the anatomical differences between spring, summer, and autumn 

 wood 3 . These differences, like the periodicity of growth in thickness, stand 

 in the closest relationship to the various functional activities at the different 

 stages of development. This is admirably shown by the fact that removal 

 of the leaves in spring, by inducing a new formation of foliage, also produces 

 a more or less distinct renewed formation of spring wood 4 . Hence the 

 annual series of changes in the structure of the wood is the result of a 

 complex correlative reaction, which we are not at present able to resolve 

 into simpler factors. Indeed to do this we must first know the causes which 

 determine the differentiation of equipotential meristem cells, some into 

 tracheides, others into tracheae, and others again into wood-parenchyma, 

 according to their position and the needs of the plant. 



It is evident that no theory can be correct which ascribes a phenomenon 

 of complex origin to the action of a single factor, and hence a brief notice of 

 the numerous existent theories will suffice 5 . It is moreover often forgotten, in 

 interpreting the results of experiments, that similar structural changes may be 

 produced by different means, so that the decreased size of cells due to a deficiency 

 of water, food, or of room for expansion does not indicate the combination of factors 

 which determines the decreased diameter of the cells of the autumn wood. 



Sachs 6 and de Vries 7 assumed that the smaller diameter of the autumn wood 



1 Miiller-Thurgau, Landw. Jahrb., 1885, Bd. Xiv, p. 881. 



2 Sachs, Vorlesungen iiber Pflanzenphysiologie, 1887, 2. Aufl., p. 347. 



3 For anatomical details see de Bary, Coinp. Anat, 1877, pp. 475, 504; Haberlandt, Physiol. 

 Anatom., 1896, 2. Aufl., p. 513. 



4 De Bary, 1. c., p. 529 ; Kny, Verb. d. Bot. Vereins der Provinz Brandenburg, 1879; Jost, 

 Bot. Ztg., 1893, p. 115; Biisgen, Waldbaume, 1897, p. 93. Similar rings occur in the wood of 

 tropical trees subject to a resting period, and also in the stems of certain Rhodophyceae (Jonsson, 

 Beitrage z. Kenntniss d. Dickenwachsthums der Rhodophyceen, 1891, p. 31 (repr. from Lunds 

 Univ. Arsskr., Vol. XXVll)). 



6 A summary is given by Schwarz, Physiol. Unters. iiber Dickenwachstum und Holzqualitat 

 von Pinus sylvestris, 1899, p. 235; Montemartini, Accrescimento delle piante, 1897, p. 17 (repr. 

 from Atti d'Istituto Botanico d. Pa via). On excentric annual rings cf. Schwarz, 1. c., p. 161 ; 

 Hartig, Centralbl. f. d. gesammte Forstwesen, 1899, Heft 7. 



6 Sachs, Lehrb. d. Botanik, 1868, I. Aufl., p. 409. 



7 De Vries, Flora, 1872, p. 241 ; 1875, P- 97 J l8 7^> P' 2 J De Influence d- 1- pression s. 1. 

 structure d. couches ligneuses, 1876. 



