60 Wisconsin Academy of Sciences, Arts, and Letters. 
the bark is not distended by low temperature during a dormant 
season. In another paper he 120 added that in accordance with 
the bark-pressure hypothesis the wood cells in roots ought to be 
small while as a matter of fact they are large. On the other 
hand he held that the changes in the radial diameter of cells 
from spring to fall can easily be explained by assuming the 
presence in them of highly osmotic substances, which induce a 
high hydrostatic pressure and as a result give rise to large cells 
in spring, while toward the end of the radial-growth period the 
hydrostatic pressure in differentiating cells is reduced owing 
to a reduction of the osmotic pressure in them. By using solu¬ 
tions of glycerine as plasmolysing agents Wieler 121 found that 
osmotic pressure in herbaceous plants was less than that in the 
living wood and ray cells of trees where it ranged from 13 to 21 
atmospheres. No difference was found, however, between the 
osmotic pressure in differentiating wood vessels and of that in 
the cambium cells. He thought that the walls of differentiating 
spring-wood cells are more distensible than those of summer 
wood owing to their lower cellulose content. 
Seasonal variation in the available elaborated food as the 
cause of “annual” rings: —After years of intimate study of 
forest trees Hartig 122 concluded that since radial growth begins 
in spring under suboptimal environmental conditions and while 
the new leaves are very small or the buds are just bursting, the 
nutritive conditions of the cambium must also be suboptimal 
and for that reason the spring wood has thin cell walls. As the 
season advances the leaves attain full size which in connection 
with the accompanying seasonal changes are conducive to the man. 
ufacture of the larger quantities of organic foods which, accord¬ 
ing to Hartig, are responsible for the production of the thicker 
walled summer-wood cells. It is held that the chief difference be¬ 
tween spring and summer wood consists essentially in the thick¬ 
ness of the cell walls and that the improvement in the nutrition of 
the cambium from early spring until the later summer is re- 
120 Russow, E. liber den Inhalt der parenchymatischen Elemente der 
Rinde vor und wahrend des Knospenaustriebes nnd Beginns der Cam- 
biumthatigkeit in Stamm und Wurzel der einheimiseben Lignosen. 
Sitzungsber. Naturfor. Ges. Dorpat. 6: 388-89. 1884. 
121 Wieler, A. Beitrage zur Kentniss der Jahresringbildung und des 
Dickenwachstums. Jahrb. Wiss. Bot. 18: 70-132. 1887. 
122 Hartig, R. Ein Ringlungsversuch. Allgem. Forst-u. Jagd-Zeit. 
65: 365-73; 401-410. 1889. 
