GROWTH AND VARIABILITY IN HELIANTHUSi 
H. S. Reed 
The growth and final size of organisms are known to exhibit greater 
or less deviations from the mean for any group or population. 
Growth may be considered as a function of two variables. The first 
is the genetic constitution of the individual, and the second is the resultant 
of all those factors w^hich make up what is commonly called the environ- 
ment of the organ^'sm. The factors of the first group are essentially internal, 
those of the second group, essentially external. 
The variability in the growth rate and in the final size of the organism 
is a question of keen interest. Much attention has been paid in the past 
to the extent and nature of the variability in mature organisms, but few 
studies have been made upon the question of variability during the grand 
period of growth. 
A brief consideration of the problem convinces one that it is not sufifi- 
cient to plant seeds of known weight and to measure the mature individuals 
at the end of the growing season. Many questions must necessarily be 
unanswered by the result of such determinations. For example, one imme- 
diately wishes to know whether the smaller individuals in the population 
have been smaller from the beginning, or whether they were normal in size 
for a time and came to maturity early; while others of the same, or of similar 
size, continued to grow later and finally attained more than average size. 
While we recognize variability in the size of plants, we also need to 
know whether a plant or a group of plants exhibits a deviation from the 
theoretical mean of the population greater or less than that to be expected 
upon the basis of pure chance; in other words, are the variations purely 
chance variations, such as might be due to the effect of purely casual factors 
of environment, or are they so characteristic as to indicate that they are 
due to something else? 
The studies embodied in the present paper are based on measurements 
of a group of sunflowers {Helianthus annuus L.) grown for the purpose on 
the grounds of the Citrus Experiment Station, Riverside, California. 
They were grown on a small piece of tolerably uniform soil to which water, 
sufficient to maintain satisfactory soil moisture conditions, was applied 
every seven days. The plants grew from the middle of May to the middle 
of August, during a time when heat and light were ample for plant growth. 
As soon as the plants had reached an average height of more than ten centi- 
^ Paper 57, University of California, Graduate School of Tropical Agriculture and 
Citrus Experiment Station, Riverside, California. 
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