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The plastochron index (P1) is a measure of plant growth and can be used to determine growth rate, based upon appearance of successive leaves on the axis of the plant. P1 should under ideal growth conditions be a regular event and should be predictable with a relatively small error of a few hours. P1 has been variously calculated in peas, and each method reported has had with it a number of problems that do not allow for reasonable prediction of P1 Internode length varies greatly and is dependent upon the variety, which may be short- or long-stemmed; thus this parameter is not ideal for determining growth rate or plant age. This paper reports our findings on P1 using the average length of the first pair of leaflets on each node. Early leaflet growth in peas occurs exponentially and the early stages of growth of successive pairs of leaflets occur at the same relative growth rate. Given that growth of leaflets during early development can be measured successfully, we propose the use of leaflet growth as a measure of the plastochron index in peas. Our results suggest that plant age is best expressed using the plastochron index, which is a measure of the time interval between the initiations of successive events - in the case of peas, of successive pairs of leaflets. [ABSTRACT FROM AUTHOR]