under the Influence of Injuries . 531 
due to injury may vary from o to 7o°/ o of the normal rate. It 
is evident that some plants respond much more readily to the 
influence of an irritation than others. This accounts in part 
for the great variation in different species. It appears how- 
ever that an injury has two opposite effects upon growth, one 
of acceleration, the other of retardation. It is clear that these 
opposing tendencies may be so nearly equal that, although 
the plant is a sensitive one, no appreciable change in the rate 
of growth can be observed. If the tendency of the irritation 
to retard the growth is weaker or of shorter duration than is 
the tendency to acceleration, the plant will soon show signs of 
acceleration and vice versa. It is seen from the tables that 
the retarding influence is stronger than the accelerating 
influence under a severe injury, but that the retarding in- 
fluence diminishes more rapidly than does the accelerating 
influence. It is impossible to determine if the retarding 
influence ever ceases entirely, or if the highest degree of 
acceleration is ever reached. We can only say that the 
amount of variation in growth is the difference between the 
accelerating and the retarding tendencies. If these opposing 
tendencies are in a state of equilibrium, the growth will be 
constant from period to period. The activities of growth 1 
may serve as an irritant to disturb this state of equilibrium, as 
shown by the almost constant variation in the rate of growth 
of normal plants. 
Summary. 
A single irritation produced by cutting or splitting the 
shoots or roots or removing the leaf-tips of seedlings tends to 
produce a change in the rate of growth of the injured and of 
the uninjured parts. 
If the injury is slight, signs of an acceleration in the rate 
of growth will be apparent in from six to twenty-four hours, 
and will continue for from one to several days. If the injury 
1 Pfeffer, Sludien zur Energetik der Pflanze, 1892. 
