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. tJber den Inhalt der parenchymatischen Elemente der 

 Rinde vor und wahrend des Knospenaustriebes und Beginns der Cam- 

 biumthatigkeit In Stamm und Wurzel der einheimischen Lignosen. 

 Sitzungsber. Naturfor. Ges. Dorpat. 6:388-89. 1884. 



121 Wieler, A. Beitrage zur Kentniss der Jahresringbildung und des 

 Dickenwachstume. Jahrb. Wiss. Bot. 18: 70-132. 1887. 



123 Hartig, R. Eih Ringlungsversueh. Allgem. Forst-u. Jagd-Zeit. 

 65: 365-73; 401-410. 1889. 



