# Plastochron

The plastochron is used to describe the leaf development of a plant, here a nettle

A plastochron is a unit used to describe the time sequence between the appearance of two consecutive leaves on the same axis or the development of other plant organs.

The Plastochron Index (PI) was developed by Ralph O. Erickson and Francis J. Michelini and first presented in 1957 in the American Journal of Botany . It is used in botany as a unit of measurement to describe the growth of a plant and to identify and compare critical growth times. In ecology it can be used to compare succession stages of plant communities . The measurement accordingly allows the age to be determined in the form of a separate time unit for the plant based on the measurement of morphological features.

## description

The background for the development of the plastochron is the consideration that the shape and size of a plant changes over the growth period and the chronological measurement of time is not sufficient to adequately describe this change. The units of time calculation must be consistently applicable for the description of the plants in a population . The conventional time calculation cannot be used optimally in these cases, as it cannot take into account statistical deviations in the beginning of seed maturity and the development of the plant and thus leads to a very wide spread in the measurement.

In the case of the plastochron index, on the other hand, the plant itself is measured with its organs and serves as a timer:

${\ displaystyle PI = n + (\ ln L_ {n} - \ ln R) / (\ ln L_ {n} - \ ln L_ {n + 1})}$

This is the sequential number of organs for which the plastochron index is to be calculated, which increases with the growth of the plant and thus along the stem axis. is the reference length of the organ and is the length of an organ that corresponds in length to or is slightly longer and is an organ that is slightly shorter than . When looking at the plastochron index of a seedling, the cotyledons are usually considered to be. ${\ displaystyle n}$${\ displaystyle R}$${\ displaystyle L_ {n}}$${\ displaystyle R}$${\ displaystyle L_ {n-1}}$${\ displaystyle R}$${\ displaystyle n = 0}$

The calculation is based on several assumptions:

1. Organ growth is exponential
2. the development of the individual organs grow at the same relative growth rate
3. the successive plastochrones must be the same, since they result from the comparison of the lengths of successive organs.

## supporting documents

1. Ralph O. Erickson, Francis J. Michelini: The Plastochron Index. American Journal of Botany 44 (4) April 1957; Pp. 297-305. ( JSTOR )
2. a b Roger D. Meicenheimer: The Plastochron index: Still useful after nearly six Decades. American Journal of Botany 101 (11), Nov. 2014; Pp. 1821-1835. doi : 10.3732 / ajb.1400305