Polytomy (Polychotomy)

Polytomy [ Botany ]	Dictionary of botanic terminology - index of names
Synonym: Multi-headedness, Forma polytomica Adjective: Polytomous or polytomic (Polychomous) Adverb: Polytomously (Polychotomously) Noun: Polytomousness (Polychotomousness)

A polytomy is the division of an apical meristem into several parts.

Photo 1: Rebutia heliosa
forma polytomica
(An inherited never-ending stable form polytomy)

Mammillaria theresae forma polytomica
(Derived from crest defasciation)

The polytomy (multi-headedness) is a type of fasciation connected with a sporadic (or continuous) division of an apical meristem into several smaller apexes from which a corresponding number of shoots appear and develop simultaneously.

The simplest case of polytomy is the dichotomous division ( division in two ) that frequently takes place among individuals of a number of plants comprising succulent and cacti. (e.g. Strombocactus disciformis, Mammillaria perkinsonii and others) In reality there is no fundamental difference between division of apical meristem into two or more parts.

Dichotomy and polytomy may be inherited or non-inherited. But most often dichotomy occurs at species that have the inherited propensity for it.
In general polytomy can occur at different stages of ontogenesis, not only at generative phase but even more often at juvenile phase without any outer stimulus.

Frequently a polytomous plant is the result of a defasciation ( For example in Mammillaria Theresae, Euphorbia obesa polytomica or in Strombocactus disciformis)

It is believed that in Cactaceae family the dichotomy should not be considered an atavism because it is found only at advanced, in terms of evolution, species and not at all the stages of ontogenesis. In fact polytomy was never found in the most primitive subfamily Peireskioideae. Also the articulated epiphytes and representatives of Opuntioideae subfamily never have real dichotomy

Compare with: Dichotomy, Cristation

Polytomy [ Cladistics - Taxonomy ]

In cladistics, a polytomy refers to a node of a cladogram with more than two descendant branches (i.e, sister taxa)

In contrast, any node that has only two immediate descendents is said to be resolved. Polytomy refers to a situation in which data is insufficient to resolve it down to two branches, which may be interpreted as a multiple speciation event. A "soft" polytomy, on the other hand, refers to a conflict between two resolved cladograms.

A cladogram in which all internal nodes are resolved as fully resolved (below left), whereas a cladogram that has a mixture of resolved nodes and polytomies as partially resolved (below center). A cladogram that is completely unresolved is termed a star tree (below right).

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