TEACHERS DEPARTMENT. 
85 
light with a horizontal microscope. No illumination of the roots 
was permitted. It was found that there is a rhythm of elonga¬ 
tion conditioned solely by internal factors. The primary maxi¬ 
mum was found to be about 4 or 5 p. m. and the secondary maxi¬ 
mum about 7 a. m. The primary and secondary minima were 
found to be at 11 p. m. and noon respectively. 
The relation between cell-division and elongation is very defi¬ 
nite. The rhythms agree in time but always occur in opposite 
directions, i. e., when elongation is at a maximum, cell-division 
is at a minimum and vice versa . The following composite curve 
deduced from all the observations made shows this in a graphic 
manner. 
9 hi 3 5 7 9 /1 / 3 5 7 9 
p. m. a. m. 
Fig. 39. Composite curves of cell division (I) and elongation (II). 
(By courtesy of Torrey Botanical Club.) 
It would seem that when the cell is dividing and the new cell 
wall forming, the energy of the cell is diverted from its other 
growth processes and elongation is arrested. 
Rhythmic activity resulting from the uniform action of stimuli 
is quite universal and it may be that here in the root where ex¬ 
ternal conditions are practically uniform we are dealing with a 
ryhthm not directly related to its environment, but which results 
from the activity of the root itself, i. e., it is internal in its origin. 
