52 Wisconsin Academy of Sciences, Arts, and Letters. 
It seems then that although trees having annual or more prop¬ 
erly 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 
Vries 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 
106 1. e. 
107 Sachs, von, F. G. J. Lehrbuch der Botanik. 1. Aufl. 1868, p. 409. 
108 Vries, de, H. Ueber den Einfluss des Rindendruckes auf den ana- 
tomischen Ban des Holzes. Vorlaufige Mitlheilung. Flora. 33:97-102. 
1875. 
