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Parastich (Visible spirals)   [ Botany ]
Derived forms: Parastichy or
in cactus Tubercles arrangement)

Dictionary of botanic terminology
index of names

  Double visible spirals pattern in the position of a leaf (or areole) on the axis.  

A specimen of Mammillaria luethyi  parastichy number 10-16 (yellow and red parastich)


Parastich are the apparent primary and secondary spiral lines visible in the position of a leaf (or of other homologous organ ), it is also used to describe the spiral line of areoles on the stem of cactus.
Each of the patterns has a certain number of primary (yellow) and secondary (red) spirals. Botanists have classified these patterns according to these parastichies numbers. Hence, if a pattern has 10 primary (yellow) and 16 secondary (red) spirals, it is said that its parastechy numbers are (10, 16): the smallest number always goes first. Each species is characterized by a defined range of parastichy number. This number may vary and increase with time from juvenile to adult age.

Right: A stem of Lobivia famatimensis v. bonnieae, it is possible to see not only the primary (yellow) and secondary (red) parastich but also tertiary (blue) and quaternary (violet)

Almost in most (about 90%) plants the ratios encountered are ratios of consecutive or alternate terms of the Fibonacci sequence.

See also: Ortostich, Phyllotaxis





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