Grossenbacher Radial Growth in Trees. 61 



sponsible for the occurrence of "annual rings." Hartig stated 

 however, that the differences in the nutritive conditions cannot 

 account for the change in radial diameter of wood cells nor for 

 the presence of the larger proportion of vessels in spring wood, 

 and maintained that the transpiration current determines their 

 size. He suggested that the reason so little difference exists in 

 the radial diameter of spring and summer wood cells of Populus, 

 Salix, Acer, etc., is to be found in the fact that these trees con- 

 tinue producing new leaves throughout most of the radial 

 growth period 'and because they have no duramen. Since the 

 water current in trees with duramen is necessarily confined to 

 the outer layers of wood its effects on cells differentiating from 

 the cambium are thought to be more marked and therefore re- 

 sult in greater differences in the diameter of spring and summer 

 wood cells, e. g. in oaks, etc. According to Hartig, then, '"an- 

 nual" rings are primarily due to the poor nutritive conditions 

 of the 'cambium in spring being followed by a period of more 

 abundant supply of metabolized food in summer, and secondari- 

 ly to a decrease in the intensity of the transpiration current 

 toward the end of the radial-growth period. 



Wieler 123 came to a diametrically opposed conclusion regard- 

 ing the differences in the nutritive conditions about the cambium 

 in spring and summer. He thought that since the character- 

 istics of "annual" rings lie in the type of wood produced in the 

 early and late growing season and not in the succession of rings, 

 the relation of different nutritive conditions to the formation 

 of spring and summer xylem could be more easily determined 

 experimentally with herbaceous than with woody plants. This 

 was deemed permissible owing to the fact that in an examination 

 of 54 species of herbs belonging to 21 families the characteris- 

 tic reduction in the size of the xylem cells toward the end of the 

 growing season as is typical of the "annual" rings of woody 

 plants, was found in over half of them. 



Seedlings of Ricinus communis were set into the soil of one- 

 fourth to one-half liter pots in spring, well watered and given 

 optimum light and temperature conditions, but they grew slow- 

 ly and remained dwarfs. In early summer four of them were 

 transplanted to the soil in a field and three of them into good 

 soil in four liter pots. Those remaining in small pots were 



123 l. c. 



