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The climate determines the types of plants that can exist in each region, of vegetation is referred to as physiognomy.
It constitutes general stature shape and life-forms of the species comprising the vegetation and actually the classification of vegetation types has been done on the basis of physiognomy.
The species of a community can be grouped into several life-forms on the basis of general appearance and growth. The life-forms of the vegetation a certain extent indicators of the climate.
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This idea has been extensively developed by a Danish botanist Raunkiaer (1934). He considered that it is the un-favourable environmental condition which exerts a major control over the growth form and he stressed the significance of the adaptations of buds and shoot tips for overcoming adverse temperatures and survival in droughty A classification of plants on the basis of the types and kinds of perennating organs seems to be natural Raunkiaer called the kinds and types of these organs as life-forms.
Raunkiaer employed three guiding rules in the selection of life-form characteristics:
(1) The character must be structural and essential and must represent important morphological adaptation.
(2) The character must be sufficiently clear so that one can easily see in nature.
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(3) All the life-forms employed must be of such nature as could constitute a homologous system and represent a single point of view or aspect of plants and thus enable a comparative statistical treatment of flora of different communities. On the basis of these principles, he recognized live life-form groups.
These groups are as follows:
(1) Phanerophytes
(2) Chamaephytes
(3) Hemicryptophytes
(4) Cryptophytes
(5) Therophytes
Cryptogams are not included in this system:
(1) Phanerophytes:
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In these, the perennating buds are located on the twigs and branches and are exposed to the atmosphere during un-favourable periods. Plants are mostly woody (trees and shrubs), although lianas, epiphytes and some large perennial herbs can be described under this group.
According to their heights, phanerophytes are divided into the following classes:
(i) Megaphanerophytes; Perennial parts including buds are more than 30 metres in height.
(ii) Mesophanerophytes; Between 8 and 30 metres height.
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(iii) Microphanerophytes; Between 2 and 8 metres height.
(iv) Nanophanerophytes; Under 2 metres and over 25 cms tall.
The above height classes are further divided according to the conditions whether the meristematic tissue of buds are protected by bud scales or they are naked and whether the plants are evergreen or deciduous.
(2) Chamaephytes:
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Buds or shoot apices are situated on shoots that lie on or near the ground surface. Plants of this class are less than 25 cm in height but their peremiating buds are found definitely above the soil surface. During the un-favourable seasons, the buds are protected by the dead fallen leaves and on higher altitudes they are protected by snow.
Chamaephytes are divided into the following classes:
(i) Subfructicose chamaephytes:
Buds are protected by dead plant remains. Examples— Caryophyllaceae, Labiatae, Leguminosae, etc.
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(ii) Passive chamaephytes:
Shoots lie on the ground surface. Examples—Sacxifraga, Polygonum, Sedum, etc.
(iii) Active chamaephytes:
Buds lie on the ground and are protected by snow under the unfavourable conditions. Example—Vinca.
(iv) Cushion chamaephytes:
They represent transition stage from chamaephytes to hemicryptophytes. In this group the shoots are closely packed, for example, Azorella.
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(3) Hemicryptophytes:
They are found on the soil surface and buds and shoots are protected by soil and dead leaves. They are herbaceous and spread on the surface horizontally as runners. The group is divided into protohemicryptophytes, partial rosette plants, and rosette plants.
(4) Cryptophytes:
In this group, the buds and shoots of plants are underground. Reserved food material is stored in the subterranean perennating organs.
The class is divided into the following categories:
(i) Geophytes
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(ii) Helophytes
(iii) Hydrophytes
(i) Geophytes:
They are land plants with subterranean perennating buds.
They are of following types:
(I) Rhizome geophytes.
(II) Stem tuber geophytes,
(III) Bulb geophytes,
(IV) Root tuber geophytes,
(V) Root geophytes.
(ii) Helophytes:
These are the plants of water saturated soil. The leaves and flowers emerge above the ground but the rhizomes are subterranean.
(iii) Hydrophytes:
They are water plants, for example—Eichhomia, Nelumbium, etc.
(5) Therophytes:
This group includes those annual summer plants which survive under un-favourable seasons in the form of seeds. In summer, all the pats of plants die except seeds and fruits. Thus, the perennating bud is actually the embryo of the seed.
Braun Blanquet’s Classification:
Braun Blanquet (1951) modified classification of Raunkiaer and proposed a new scheme of classification of vegetation.
The scheme is as follows:
1. Phytoplankton (Microscopic floating plants):
(a) Aeroplankton (float in air)
(b) Hydroplankton (float in water)
(c) Cryoplankton (float in ice and snow)
2. Phytoedaphon (Microscopic soil flora):
(a) Aerophytobionts (aerobic) Bacteria
(b) Anaerophytobionts (anaerobic)
3. Endophytes:
(a) Endoxylophytes (Parasites of plants)
(b) Endolithophytes (Algae, Fungi and Lichens)
(c) Endozoophytes (Pathogens on animals)
4. Therophytes:
(a) Thallotherophytes (Myxomycetes and moulds)
(b) Bryotherophytes (Liverworts and mosses)
(c) Pteridotherophytes (Vascular cryptogams)
(d) Entherophytes (annual seed plants) which may be creepers (climbers or erect).
5. Hydrophytes (All water plants except planktonic forms)
6. Geophytes (Earth plants):
(a) Geophyta mycetosa (Root and tuber inhabiting fungi)
(b) Geophyta parasitica (e.g., Rafflesia, a root parasite)
(c) Eugeophytes with bulb, rhizome and root tubers.
7. Hemicryptophytes (Some thalloid algae, fungi, lichens and rooted creepers, climbers and Rosette plants).
8. Chamaephytes (Mosses, lichens, leaf succulents, cushion plants, hard grasses, semishrubs and trailing shrubs).
9. Phanerophytes (Succulent cacti, herbs, liana plants).
10. Epiphyta arboriela (Tree epiphyte).
Biological Spectrum or Phytoclimatic Spectrum:
Raunkiaer’s system is unitary being based completely on the position and protection of perennating buds during un-favourable seasons. The characters are structural, essential and adaptive. Furthermore, they provide simple basis for statistical treatment. The array of percentages of life- form classes of a flora of any area composing any floristic community is called the biological spectrum or phytoclimatic spectrum. Since each life-form class is related to the environment, the biological spectrum is direct indicator of its environment.
In biological spectrum, the life-form classes are represented by their percentages. Raunkiaer prepared a normal spectrum based on sampling of world flora using one thousand entities. The normal spectrum provides a base line from which the departure of percentages of any class in any given flora can be ascertained.
The normal spectrum has:
Phanerophytes 46%
Chamaephytes 9%
Hemicryptophytes 26%
Cryptophytes 6%
Therophytes 13%
Generally, the biological spectra are worked out and compared with Raunkiaer’s normal spectrum. The percentage of phanerophytes in different floras ranges from zero to over 74% in the tropical rain forest. The higher percentage of phanerophytes is indicative of phanerophytic climate. The higher percentage of therophytes (above 40%) indicates therophytic climate (desert). High percentage of chamaephytes (above 50%) indicates an extremely cold climate.
High percentage of hemicryptophytes in temperate forest vegetation indicates the conditions favourable for the development of extensive grassland. Geophytes (cryptophytes) are abundant in the regions of Mediterranean climate and in the broad leaf deciduous forest. The biological spectra of different areas differ from one another and there is no base line for comparison.
There are certain limitations to usefulness of biological spectrum as are indicative of climatic condition, because at certain places it does not indicate the environmental conditions, for example, therophytes are abundant in Indogangetic planes where phanerophytic vegetation should actually be dominant. Biotic agencies are the chief causes for changing the biological spectrum in a given floristic zone. Thus comparison of biological spectrum with normal spectrum may sometimes create confusions and may also lead to wrong conclusions.
As the leaves are essential part and are very much affected by climatic conditions, their shapes and sizes have been taken as important criteria in further classification of different life- forms.
Raunkiaer used leaf size as important character in classification, and classified plants into the following leaf sizes classes:
