The following points highlight the top two categories of diversity in habit of the plant. The categories are: 1. According to the Height 2. According to the Life-cycle.
Diversity in Habit of the Plant: Category # 1.
According to the Height:
They may be:
(i) Herbs,
(ii) Shrubs and
(iii) Trees.
(i) Herbs:
These are small plants with soft stems. They may be annuals, e.g., mustard, pea, rice, etc., biennials, e.g., beet, carrot, turnip, etc., perennials, e.g., canna, ginger, banana, etc.
The annuals are those herbs that attain their full growth in one season, living for few months or at the most for one year only. The biennials are those herbs that live for two years and they produce flowers and seeds in the second year after which they die off.
The perennials are those herbs that persist for a number of years. The aerial parts of such plants die every year at the end of the season but new shoots develop again from the underground stem.
(ii) Shrubs:
They are medium sized plants with hard and woody stems. They branch profusely from near the ground, and thus the plants become bushy in habit without a clear trunk. The examples are Hibiscus rosa-sinensis, night jasmine (Nyctanthes arbortristis), garden croton, etc.
(iii) Trees:
They are tall plants with a clear trunk and possess hard and woody stem and branches, e.g., mango, sissoo, nim, teak, jack, etc.
The shrubs and trees are perennials.
Diversity in Habit of the Plant: Category # 2.
According to the Life-cycle:
They may be:
(i) Annuals,
(ii) Biennials and
(iii) Perennials.
(i) Annuals:
Complete their whole life, from seed to fruit in one year or less. In some cases even in a few weeks such as in Senecio vulgaris, so that several generations may be passed through in one summer while nothing but the seeds remain through the winter. Biennials and annuals are therefore, typically monocarpic, fruiting but once.
(ii) Biennials:
Last only for two years. In the first season they produce at soil level a very contracted stem bearing a rosette of leaves. During the second season the stem elongates and bears the flowers and fruit, after which the whole plant dies, e.g., carrot, radish, turnip, etc.
(iii) Perennials:
May be woody, either tree like with one main trunk, or else shrub-like with a cluster of stems. They may also be herbaceous lying down to the ground level each winter and persisting only by underground organs. The duration of perennials is very variable.
Some herbs live only five to six years. Large trees on the other hand may take twenty five to thirty years to attain flowering. The common factor in all perennials is that they are polycarpic, i.e., they flower and fruit again and again.
Certain monocotyledonous perennials, on the other hand, are naturally monocarpic. For example, bamboos flower every twenty to thirty years and then die. Agave americana flowers once but only after hundred years producing a 50 feet inflorescence and thereafter dies.