Plant Biology - Structure: Whole Plant

GROWTH FORMS

The great variety of plant form arises from the different ways in which they grow. This can be quite difficult to describe although the difference in appearance may be obvious. There are many ways of describing plant form and this is just a beginning. More detail can be found in books such as "Plant Form: An Illustrated Guide to Flowering Plant Morphology" by Adrian D. Bell, Oxford Univesity Press, 1991. Surprisingly, few botany or horticulture books bother with this subject, although it has implications for how we use plants, how they cope with their environment and how we cn handle or control them (propagation, pruning, etc.)

One way of looking at plant form was devised by the German botanist, Raunkiaer. His terminology seems foreign, even weird but it does cover some basic distinctions in types of plants according to where they grow and how they survive through the winter. The picture represent perennial plants:

Phanerophyte ("obvious" plant): tall growing woody plants
Chamaephyte ("ground" plant): low growing shrubs (up to 20 cm or 8")
Hemi-cryptophyte ("half-hidden" plant): dies back to crown of buds at soil surface
Geophyte ("earth" plant): dies back to underground bulb/corm/rhizome/root
Helophyte ("not free" plant): grows in water but attached to bottom
Hydrophyte ("water" plant): free floating
Not shown Therophyte ("beast" plant?): annuals that persist through seeds

Other ways of describing plants are based on how much they branch, the direction in which branches grow and how far they grow. Trees will often change their branching pattern as they grow. Trees will often change their branching pattern as they grow so that quite complex shapes build up over time and it is hard to figure out how they got that way. However, this is what determines much of their ornamental value in the landscape and affects how they will respond to cutting back. Will we lose all the flowers for this year or possibly spoil the shape forever? Cutting back sometimes leads to "recapitulation": one or more shoots goes through the branching sequence from the beginning of the growth of the tree. Alternatively a cutting from a side shoot may continue the branching pattern that it had established on the original plant. Strange shapes can develop!

Here are some basic distinctions (there are many variations):

Excurrent growth: strong main stems
Plagiotropic branches: horizontal growth
Decurrent or fastigiate growth: no dominant main stem (can be combined with orthotropic or plagiotropic habit.)
Monopodial growth: no dominant main stem (can be combined with orthotropic or plagiotropic habit.)
Sympodial growth: growth continues from axillary buds, often because shoots end in flower each season.