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 4 4 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 1. c. 
