The below mentioned article provides an overview on the life forms or growth forms of plants.
Various types of plant communities resemble or differ from each other in different ways. In various communities of trees such as pine, oak, beech, etc., the communities resemble each other in that in each, the dominant and the characteristic species are trees, but they differ from each other in that these species are pine, oak, and beech respectively.
Thus plant communities may differ from each other not only in respect to their dominant species, but may also at the same time resemble each other in that the dominant species in all cases may be trees, shrubs, or herbs or in other words, are of the same life or growth form.
Thus, if we recognise behaviour of units of the vegetation, the life forms (the vegetative form of the plant body, thought usually to be a hereditary adjustment to environment; growth form) of the plants comprising these units must also be taken into account. The life forms of the various plants comprising a community play an important part, as would be readily seen, in determining its general structure.
Any classification of life forms must take into account the characteristics of a plant adjusting it to its habitat. Different classifications of life forms based on different vegetative features have been proposed from time to time but the earliest one was proposed by Humboldt (1806). He attempted to group vegetation types on a physiognomic basis, with at least some effort to express relations between environment and certain life-form groups.
Humboldt named about 15 groups of plants whose physiognomy was important in the study of landscapes:
1. Banana form, Musa, Strelitzia, etc.
2. Palm form, Cocos, etc.
3. Tree fern form Alsophila, Cyathea, etc.
4. Aroid form, Arum, Pothos, etc.
5. Conifer form, Taxus, Pinus, Picea, etc.
6. All sharp-leaf form, Araucaria, funiperus.
7. Tamarisk form, Mimosa, etc.
8. Mallow form, Sterculia, Hibiscus.
9. Liana form, Bauhinia, etc.
10. Orchid form, Epidendron, Cattleya, etc.
11. Cactus form, Opuntia, Cereus, etc.
12. Casuarina form, Casuarina, Equisetum.
13. Grass form, Andropogon, Panicun, Scirpus, etc.
14. Moss form, Bryum, Sphagnum, Polytrichum, etc.
15. Lichen form, Cladonia, Gyrophora, etc.
The following classification (Raunkiaer), which came much later, based on a single principle—the position of the perennating vegetative bud in relation to the ground level—has proved to be of great value and has received considerable world-wide use:
1. Parasites and Epiphytes.
2. Plants with succulent habit.
3. Climbers and Twiners.
4. Trees and shrubs—phanerophytes (PH) (from phaneros—visible):
(a) Tall trees (more than 30 metres in height)—megaphanerophytes (Mg) (from mega—large);
(b) Trees (between 30 metres and 8 metres)—mesophanerophytes (Ms);
(c) Trees and shrubs (between 8 metres and 2 metres)—microphanerophytes (Mc) (from micro—small);
(d) Shrubs (between 2 metres and 25 cm)—nanophanerophytes (N) (from nanus—dwarf);
(e) Undershrubs or herbs with vegetative buds not more than 25 cm above the surface of the soil—chamaephytes (Ch) (on the ground);
(f) plants with buds on the surface of the soil—hemicryptophytes (H) (from hemicrypto—half-hidden);
(g) Plants with buds buried deep in the soil on underground stems—geophytes (G) (from geo—in the earth).
5. Annual herbs—therophytes (Th) (from theros—summer).
6. Water and marsh plants—plants with buds under water—hydrophytes (from hydro—wet) and helophytes (from helo—marsh).
All forest communities of the tropical regions are dominated by phanerophytes, the deserts by nanophanerophytes and chamaephytes (ephemerals in some deserts are therophytes), grassland by hemicryptophytes and much of the cold arctic and alpine vegetation by chamaephytes.
Communities inhabiting loose and soft soil are marked by the abundance of geophytes. Therophytes are abundant on all open soil habitats, on seashores and in all watery places are early phases of plant succession.
An interesting aspect of the distribution of life forms—the list of percentages of different classes of life forms in the total flora of a climatic region—biological spectrum —is clearly related to the prevailing climate.
A very interesting example of variations in life form adopted by some species of plants due to changes in the environmental conditions, particularly climate, is found in the Eastern Himalayas. The flaming red-flowered Rhododendrons which are tall trees (megaphanerophytes?) at altitudes between 1800-2400 metres become strikingly dwarf, microphanerophytes at alpine altitudes of 3600-4200 metres. The effect is mostly thermal, no doubt, for near freezing temperatures generally prevail at high altitudes for most of the year but preponderance of ultra-violet radiations in sunlight (compared to light at lower altitudes) and high wind velocity might also be significant contributory factors.