Editing Phyllotaxis (section)

== Leaf arrangement ==
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The basic [[leaf#Arrangement on the stem|arrangements of leaves on a stem]] are '''opposite''' and '''alternate''' (also known as '''spiral'''). Leaves may also be '''[[Whorl (botany)|whorled]]''' if several leaves arise, or appear to arise, from the same level (at the same [[Node (botany)|node]]) on a stem.

[[File:Veronicastrum virg album C.jpg|thumb|upright|''[[Veronicastrum virginicum]]'' has whorls of leaves separated by long [[Plant stem|internodes]].]]

With an opposite leaf arrangement, two leaves arise from the stem at the same level (at the same [[Node (botany)|node]]), on opposite sides of the stem. An opposite leaf pair can be thought of as a whorl of two leaves.

With an alternate (spiral) pattern, each leaf arises at a different point (node) on the stem.

[[File:Clivia-GreenPlants.ca.jpg|thumb|right|Distichous leaf arrangement in ''[[Clivia]]'']]

'''Distichous''' phyllotaxis, also called "two-ranked leaf arrangement" is a special case of either opposite or alternate leaf arrangement where the leaves on a stem are arranged in two vertical columns on opposite sides of the stem. Examples include various [[bulb|bulbous plants]] such as ''[[Boophone]]''. It also occurs in other plant [[Habit (biology)|habits]] such as those of ''[[Gasteria]]'' or ''[[Aloe]]'' seedlings, and also in mature plants of related species such as ''[[Kumara plicatilis]]''.
[[File:Lithops 2.jpg|thumb|A ''[[Lithops]]'' species, showing its decussate growth in which a single pair of leaves is replaced at a time, leaving just one live active pair of leaves as the old pair withers]]
In an opposite pattern, if successive leaf pairs are 90 degrees apart, this habit is called '''[[decussate]]'''. It is common in members of the family [[Crassulaceae]]<ref name="Eggli2012">{{cite book| first = Urs | last = Eggli | name-list-style = vanc |title=Illustrated Handbook of Succulent Plants: Crassulaceae|url=https://books.google.com/books?id=nU7mCAAAQBAJ&pg=PA40|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-3-642-55874-0|pages=40–}}</ref> Decussate phyllotaxis also occurs in the [[Aizoaceae]]. In genera of the Aizoaceae, such as ''[[Lithops]]'' and ''[[Conophytum]]'', many species have just two fully developed leaves at a time, the older pair folding back and dying off to make room for the decussately oriented new pair as the plant grows.<ref name="Hartmann2012">{{cite book| first = Heidrun E.K. | last = Hartmann | name-list-style = vanc |title=Illustrated Handbook of Succulent Plants: Aizoaceae A–E|url=https://books.google.com/books?id=7oHuCAAAQBAJ&pg=PA14|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-3-642-56306-5|pages=14–}}</ref>

If the arrangement is both distichous and decussate, it is called '''secondarily distichous'''.

[[File:Ulistnienie (Phyllotaxys opposite-decussate).png|thumb|upright|A [[Decussation|decussate]] leaf pattern]]
[[File:Crassula rupestris-PICT3087.jpg|thumb|[[Decussation|Decussate]] phyllotaxis of ''[[Crassula]] rupestris'']]

The whorled arrangement is fairly unusual on plants except for those with particularly short [[internode (botany)|internodes]]. Examples of trees with whorled phyllotaxis are ''[[Brabejum stellatifolium]]''<ref name= "Marloth4">{{cite book | last = Marloth | first = Rudolf | name-list-style = vanc | title = The Flora of South Africa | date = 1932 | location = Cape Town & London | publisher = Darter Bros., Wheldon & Wesley }}</ref> and the related genus ''[[Macadamia]]''.<ref name= "RHSDict">{{cite book | last = Chittenden | first = Fred J. | name-list-style = vanc | publisher = Royal Horticultural Society | title = Dictionary of Gardening | location = Oxford | date = 1951 }}</ref>

A whorl can occur as a [[anatomical terms of location|basal]] structure where all the leaves are attached at the base of the shoot and the internodes are small or nonexistent. A basal whorl with a large number of leaves spread out in a circle is called a [[Rosette (botany)|rosette]].