The Annual and Deciduous Habits

It is generally accepted that deciduous trees and shrubs shed their leaves in order to escape either the rigours of winter in the temperate zones, or the desiccation of a dry season in the tropics. And that, for the same reasons, annual plants die off completely, except for their well protected, dormant seeds. Many perennial herbs also die off above ground, and survive an adverse season solely as underground organs, such as tubers, corms, or bulbs, that are capable of regenerating new plants with a return to favourable growing conditions. These various dormant organs are also capable of surviving fire.

It is much less generally appreciated that these deciduous and annual habits also have an important survival advantage in terms of parasitism, and that they provide a plant pathosystem with sequential discontinuity. In its turn, this sequential discontinuity permits both the evolution, and the functioning, of a gene-for-gene relationship, and the stabilising effects of a system of locking.

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