The Life Span of Plants

How long does a plant live? Can environmental conditions such as soil and climate have any influence and how much can individual plants deviate from the normal pattern? All these are questions which are often asked. The first is the most difficult to answer, and to do so with any sense it is necessary to look at the three broad groups into which higher plants are divided. These are annuals, biennials and perennials.

Annuals have the shortest life span, completing their whole cycle from seedling to new seed well within twelve months. Many germinate as the weather warms up in spring, taking advantage of the lengthening days and increasing warmth to grow rapidly and complete the process of flowering and fruiting before the chill of autumn arrives. Others gain an advantage by germinating in autumn and over-wintering as a small seedling ready to begin growth as soon as spring comes. Many of the most successful annuals are capable of following either pattern and their seeds can germinate at any time when the weather is mild enough. Not surprisingly, many weed species fall into this category. Whatever their timing, however, annuals always complete their life cycle within one calendar year.

Biennials take two years to complete their lives, normally germinating and forming a young plant in the first season, then flowering and fruiting in the second. If for some reason the young plant does not reach sufficient size to be ready for flowering in its second year, then it will often live through another winter and complete its cycle in the third year. Perhaps the best term for these plants which live for more than a year and then die after flowering is monocarpic — literally ‘once seeding’. True biennials are all monocarpic, but the term can also be applied to perennial plants that produce rosettes which die after flowering, but which may at the same time leave behind offsets as a form of vegetative reproduction. The Century Agave (Agave americana) is an example of a plant with monocarpic flowering rosettes, so are the houseleeks (Sempervivum).

The true perennials can live from three to three thousand years depending upon species and conditions. They have to face the problem of surviving inclement weather so as to flower and fruit again in successive years. Herbaceous plants die down to the ground after fruiting, passing the winter as bulbs, rhizomes, tubers or dormant crowns. For them the soil provides protection from all weathers above. Other non-woody perennial plants remain leafy throughout the winter. Among these are many which form low rosettes closely pressed to the ground and, in severe climates, are safely covered in snow during the worst weather. Others remain with leafy stems and are mostly plants of milder climates, unable to survive winters of the severity normally encountered by most of the truly herbaceous species.

Woody trees and shrubs are also true perennials and in cold climates their problem is to cut down transpiration in winter when there is not enough water available at the roots to keep the leaves supplied. Many species surmount this difficulty by adopting a deciduous habit, losing their leaves in autumn. Leaf fall is not a matter of leaves shrivelling with the onset of cold weather but is part of the physical make-up of the plant being triggered off by the decreasing day-lengths of autumn.

Leaves are not broken from the tree but fall because they have been cut off from all contact with the vascular bundles of the adjoining stem. This separation is prepared for long before the actual leaf fall takes place; as soon as a leaf is full grown, in fact, it is possible to see with a microscope that the vascular bundles are thinner where the leaf stalk or petiole joins the stem. At the same point the thickened sclerenchyma and collenchyma cells are few, and the parenchymatic cells which are present have a thicker cytoplasm. As days shorten, the walls of these cells break down and a protective layer, at first of fatty tissues, then of cork cells, forms where the break will occur. When this protection is complete, the vascular bundles are effectively blocked and the leaf no longer has any living connection with the tree. If a branch is cut or broken from a tree before the corky layer is formed, the leaves will not fall but remain dried upon the tree. Extreme drought, shortage of a vital mineral or a similar adverse condition can bring about leaf fall out of season, the factor which triggers off leaf loss in each case being the tree’s need to conserve its existing water content.

With the onset of colder conditions in autumn, chlorophyll in the leaves of deciduous plants breaks down and the green colouring disappears. The yellow and red pigments remain, however, leaves with a high content of carotene or xanthophyll take on yellow colouration and those with strong anthocyanin become red. These are the autumn colours which are such a feature of woodlands, especially when hot, sunny summers have concentrated the amount of pigment present.

For a while the woods are painted with colour, but by now the leaves have only a tenuous attachment to the tree and the first gale and strong rain sweeps them to the ground. If autumn is calm, the leaves will hold longer but at the first sharp frost, moisture seeping in at the base of the petiole will freeze and the expansion will be sufficient to break the leaf free.

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