Nuttonson — 143 — Phenological Data 



III. Measurements of the interval between vegetative and reproductive stages, 

 as well as that between planting or emergence and ripening, suggest a multiple of 

 the average length of day and the summation of day-degrees as the least variable 

 numerical expression. The use of days or day-degrees alone as a unit of measure- 

 ment provides mathematical expressions of greater variability than that of the multiple. 



IV. Blossoming or heading in some varieties appear to be associated with a re- 

 sponse to more than one environmental factor, namely, to a joint effect of the average 

 length of day and the summation of day-degrees of the vegetative stage. 



V. An analysis of the data used in this work suggests that the temperature must 

 be taken into account in studies dealing with the relation of certain plants to light 

 duration. It also suggests that the transition from vegetative to reproductive stage, 

 as well as the transition from initiation of growth to biological or market maturity, 

 may occur with some plant varieties under a number of combinations of temperature 

 and length of day conditions. 



VI. Summation of day-degrees required by all the horticultural varieties pre- 

 sented in this paper appears to increase in a southward direction, that is, with the de- 

 crease of the average length of day duration. 



