398 
PHYSIOLOGY: H. S. REED 
Proc. N. a. S. 
was thus uniform, and since the trees were periodically irrigated through- 
out the season, it is believed that conditions necessary for fairly uniform 
growth were afforded. The trees, being so young, produced no fruit and 
consequently growth was not influenced by that activity. The selected 
shoots were marked with a line of India ink ten centimeters from the 
growing point and all measurements were made from this as a base line. 
Most of the shoots produced several lateral branches during the season. 
The length of these laterals was disregarded as far as these studies are 
concerned. The propriety of this procedure may be questioned, yet it is 
difficult to see any valid objection thereto, because in measuring the in- 
crease in length of a growing shoot we are measuring the catalytic activity 
of a group of meristematic cells in the apical bud of that shoot. Unless 
the lateral branches of a shoot have some modifying effect either upon the 
growth catalyst, or upon the supply of material upon which the catalyst 
acts, it is difficult to see how the presence of the lateral shoots can modify 
the growth rate of the main branch. Since we know that removal of the 
lateral branches of a shoot retards rather than accelerates the length- 
growth of the main shoot, we are not warranted in assuming that their 
presence has a retarding influence upon the growth of the main shoot. 
The Course of Growth of the Apricot Shoots.'^ — If the growth of a branch 
is similar in its rate to some form of autocatalytic reaction, we should 
expect the length of the branches to be determined by some sort of a 
unimolecular reaction formula which shall involve the time and a growth 
constant. 
The most generally useful equation to express this sort of a rate is that 
of autocatalysis, 
dx/dt = kx{a~x) (1) 
in which a represents the final size of the plant, x is the height (or size) 
at any time, t, and k is the velocity constant of the reaction. A paper 
recently published (Reed and Holland, 1919) has shown that the growth 
rate of Helianthus is well expressed by this equation, but it does not 
necessarily follow that the growth rate of individual shoots on a perennial 
tree will follow the same type of growth as the axis of an annual plant 
like the sunflower. 
Inspection of the observed values shows that there were three quite 
distinct cycles of growth in the growing season. The existence of these 
cycles may be seen by inspection of the graph in figure 1, but better by the 
observed increments in table 4. The large weekly increments in the first 
few weeks of the season diminish to a minimum value at the 9th week; 
then for a few weeks are larger, diminishing again to the 18th week; they 
increase to the 21st week and then decline to the end of the season. The 
season's growth, therefore, consisted of three cycles of growth each of 
about 9 weeks duration. These cycles were quite apparent during the 
