53 ° Townsend . — The Correlation of Growth 
In some of the experiments a distinct difference in the rate of 
growth was observed in six hours, while in other cases no 
change could be seen within twenty-four hours after injury. 
Even among plants of the same species and under the same 
conditions there were individual differences in the time 
required for the variation in the rate of growth. It was rare 
that a change in the rate of growth which could not be 
accounted for by individual differences was noticeable in less 
than six hours. The duration of the retardation or accelera- 
tion depended largely upon the degree of injury. If the 
injury was sufficiently severe, the retardation continued 
throughout the life of the seedling ; while with a less severe 
injury the retardation lasted only from one to three days, and 
was often followed by a period of acceleration (Table II, Series 
II and III). If the injury was slight, it was usually followed 
by acceleration without an intervening period of retardation 
(Table VI). The influence of the irritation upon the seedlings 
was usually much more gradual than upon the older plants. 
The total period covered* by a variation in the rate of growth 
due to a single irritation varied from one to twelve days, and 
depended upon the condition of the individual plant and upon 
the degree of injury. It was possible for the retarding and 
accelerating influences to neutralize each other so that the 
final result was the same as if no injury had been inflicted. 
Indeed it was possible for the final result to indicate a slower 
growth than the normal even though the injury was followed 
by a period of acceleration, provided that a strong retardation 
followed or preceded the acceleration. 
With Phycomyces a reduction in the rate of growth took 
place immediately after cutting, as shown in Table XIV. 
The retardation was evidently due to a change of turgor 
rather than to an irritation produced by cutting. At any rate, 
it is impossible to know what part the irritation plays in 
these experiments so long as the influence of turgor is not 
eliminated. 
Extent of variation in growth . — It is apparent from a study 
of the preceding tables that a change in the rate of growth 
