52 Wisconsin Academy of Sciences, Arts, and Letters. 



It seems then that although trees having annual or more prop- 

 rrly radial-growth rings are distributed all over the arborescent 

 world, one or more factors of their envionment must be effective 

 periodically in order that marked zonation occur. The more or 

 less regular recurrence of cold or dry seasons are the factors 

 usually noted in connection with periodically recurrent vegeta- 

 tive seasons, but doubtless any other recurrent environmental 

 factor influencing growth may also affect zonation, e. g., periodic 

 variation in the supply of inorganic foods as was suggested by 

 Klebs. 106 It should be noted, however, that wood zonations re- 

 sulting from recurrent dry periods of the tropics even in decid- 

 uous trees are not as marked as those occurring in temperate 

 zones where the dormant period is chiefly due to seasonal varia- 

 tions in the temperature and where consequently a greater sea- 

 sonal change occurs in the bark pressure. 



The causes of the formation of radial-growth rings have been 

 studied mainly in the north temperate zone and, therefore, ex- 

 planations are largely based on the environmental factors that 

 seem to be operative in that region. Seasonal changes in bark 

 pressure, in the supply of metabolized food to the cambium, and 

 in the rate of transpiration have been either separately or in 

 partial combination advanced as explanations for the occurrence 

 of the large-celled spring-wood alternating with small-celled 

 summer-wood. 



The bark-pressure hypothesis: Sachs 107 seems to have been 

 the first to suggest that the difference between spring and sum- 

 mer wood may be due to a difference in the bark tension or pres- 

 sure obtaining in spring and summer. The idea was then tested 

 experimentally by de Vries 108 with the result that Sachs' hy- 

 pothesis seemed to have been sustained. The experiments by de 

 Yries consisted in making some longitudinal slits in the outer 

 bark of various trees in spring and of applying ligatures to the 

 stems of others. On the following winter it was found that only 

 about one-half as many cells had been produced under the liga- 

 tures as occurred on other parts of the past season 's ring ; while 

 in the regions where the outer bark had been slit the number of 



108 1. e. 



107 Sachs, von, P. G. J. Lehrbuch der Botanik. 1. Aufl. 1868, p. 409. 



IDS Vries, de, H. Ueber den Einfluss des Rindendruckes auf den ana- 

 tomischen Bau des Holzes. Vorlaufige Mitlheilung. Flora. 33:97-102. 

 1875. 



