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Arid zone habitats have a fascination of their own. The form which the vegetative body of the plant produces as the result of all the life processes which are affected by the environment has been designated the “vegetative from”, “growth form” or “life from”. Life forms of plants reflect the major feature of the climate. It further provides a sound basis of natural ecological classification.
The position of a plant in Raunkiaer (1934) system is determined by the location and protection of the perennating organs (renewal hud) during the unfavourable season, that is, during the dry, hot summer or the cold winter. On this basis Runkiaer distinguished: phanerophytes, chamaephytes, hemicryptophytes, cryptophytes, and therophytes. The crytogams are not included in this system.
Raunkiaer originally intended his life forms to be used only in the ecological characterization of definite climatic regions and in the description of plant climates. For this purpose he determined the proportion of each life form in the flora of the world as a whole, and arranging classes in a manner somewhat analogous to the arrangement of the colours of the spectrum, he designated the result a “normal biological spectrum”.
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Raunkiaer thought that five of the classes are to be most significant and are designated by an appropriate letter following the name. These classes are phanerophytes (F); or (Ph), chamaephytes (Ch); hemicryptophytes (H); geophytes (G) (or cryptophytes); and therophytes (Th). The biological spectra for the world as a whole is the normal spectrum.
In the table 10.1 life from studies of arid zone by Das and Swarup (1952) and Mertia and Bhandari (1980) are compared with Raunkiaer (1934) normal spectrum. The comparison shows that the life form spectrum changes progressively in the extreme arid region, the change being a shift towards emphasis on the more protected life-form classes.
An analysis of the table 10.1 reveals that the therophytes form the major bulk of plant components of the extreme arid region of the Indian arid zone. The therophytes, is of frequent occurrence in the arid and the semi-arid zones and under certain extreme climatic conditions, is the only one capable of survival.
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The vegetative cycle of this life form is very short (Ephemerophytes) and 4 to 5 weeks in many cases are enough to cover germination, growth, flowering, seeding and death. A short wet period, every few years, is all that such species need to Survive, in such regions.
Therophytes are annuals which complete their life cycle, from germination to ripe seeds, within a single vegetative period. Their seeds or spores survive the unfavourable season under the substratum. Due to this habit and their mobility they are widely distributed, even in the unfavourable hot, dry regions of the earth.
Ecological Significance of Life Forms:
The biological spectrum of an association sometimes reveals interesting and unsuspected ecological relations. The spectra of life forms give a direct impression of the climatic peculiarities of the various phyto-geographic regions of the earth.
The better a species can adjust itself to the short vegetative season, the more rapidly it completes its life cycle the more completely is it suited to live and spread in the high arid zones.
Each plant community consists of a definite group of life forms. Each habitat favours certain groups of life forms and almost excludes others. The more extreme the habitat conditions the sharper, the selection and the more pronounced are the ecological characteristics of the life forms. For this reason biological spectra of decidedly pioneer associations, best reflect the ecological relations of the habitat.
