Branching Patterns
Below are several types of branching patterns found throughout the plant kingdom
These patterns can be found in stems, leaves, and vein branching. All of these patterns are created through apical cells that divide or split during mitosis.
Dichotomous Branching
This form of growth is found in plants that possess an apical cell that bifurcates to produce two branches of equal or unequal size (e.g. early polysporangiophytes, lycophytes, trimerophytes, equisetophytes, etc.)
With dichotomous branching, both the sizes of the daughter axes and the angle of divergence from the parent axis need to be considered
Isotomous branching
"Isotomous" means equal splitting
The resultant (daughter) branches are of equal size/vigor and also diverge at similar angles from one another
e.g. The apical cell splits to produce two new axes that are similar in diameter to one another, but probably smaller than the parent axis. The angle of divergence is 45 degrees for both new axes.
This type of growth can be seen in Psilotum, Lycopodiales, and many Silurian and Devonian plants
Anisotomous branching
"Anisotomous" means unequal splitting
The resultant (daughter) branches may be of unequal size/vigor, but more commonly daughter branches diverge at different angles from one another.
e.g. One branch ("A") at a slight divergence from the main, such as 10 degrees, but the other axis ("B") is much more divergent, such as 60 degrees, creating an "overtopped" appearance. Frequently the daughter axis with greater divergence, "B", is also smaller in diameter to the less divergent daughter axis, "A".
This type of growth can be observed in Selaginella, as well as many Devonian plants
Pseudomonopodial branching
Pseudomonopodial means false monopodial growth (see below). This type of branching creates a resultant form that similar looking to true monopodial growth
This is an extreme version of anisotomous branching, in which the daughter branches are very different in both size/vigor and angle from one another.
The resulting appearance is such that one daughter axis, "A", continues the upright, perpendicular growth of the plant. It has the same angle of growth as the parent axis, such that it seems to continue the growth of the main axis
The other daughter axis, "B", is much more divergent creating an appearance of side branches. These axes are more divergent in angle from the main axis and "A" axis, but also smaller in diameter
This type of growth can be observed in Equisetum, "trimerophytes", and many other Devonian plants
This is not how true trees grow (i.e. monopodial growth - see below)
K-Branching
This is a dichotomous pattern sometimes seen in lycophytes, in which close branching creates a K-shaped group of axes
Growth from a rhizome creates dichotomous branching, in which the main axis continues in a pseudomonopodial manner
The sister axis of this branching event then quickly forks again to create two isotomous axes. __\/__
In some taxa, this latter isotomy results in an upright shoot and downward root-bearing axis (e.g., Asteroxylon, Senegelia)
Above: A reconstruction of Senegelia radicans, a lycophyte from the Early Devonian, exhibiting K-branching
Monopodial Growth
This form of growth is found in plants with a shoot apical meristem (SAM), which produces new length through growth (mitosis), and branches are created through dormant axillary buds created by the apical meristem
These axillary buds open at a later time to produce branches or branching
e.g. lignophyte plants: progymnosperms, conifers, angiosperms
Phyllotaxis
During growth/development, the SAM produces leaves (or laterals) in a specific pattern along a main stem. This pattern in which these leaves are created and arranged on the stem is called a phyllo-taxis. Below are some common examples (node=place where a leaf attaches to the stems)
Alternate
Only one leaf is attached at a node, resulting in a form that exhibits leaves that look like they "alternate" sides along the stem
Opposite
Two leaves are attached at each node, resulting in a form that exhibits leaves that are always opposite from each other on the stem
Distichous
A "two-ranked leaf arrangement" where the leaves (or laterals) on a stem are arranged in two vertical columns on opposite sides of the stem. The leaves may be inserted in an alternate or opposite fashion
Decussate
Leaf pairs or laterals are 90 degrees apart with each set.
Whorled
Three or more leaves are attached at each node
n.b. Roots usually do not show a set pattern like leaves, although the root-like stems (rhizophores) of arborescent clubmosses displayed a spiral "rhizotaxis" of roots emerging from these structures
