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We can see how modular organisms grow by taking higher plants as a good example. The fundamental module of construction above ground is the leaf with its axillary bud (the bud emerging where the leaf meets the stem) and the attendant section of stem. As the bud develops and grows, it produces further leaves, each bearing buds in their axils. The plant grows by accumulating these modules. At some stage in the development, a new sort of module appears, associated with reproduction (flowers in higher plants) and ultimately giving rise to new zygotes. Such specialised reproductive modules usually cease to give rise to new modules. The programme of development in modular organisms is typically determined by the proportion of modules that are allocated to different roles (e.g. to reproduction or to continued growth).

Depending on how they grow, modular organisms may broadly be divided into those that concentrate on vertical growth, and those that spread their modules laterally, over or in a substrate. Among plants that mostly extend laterally, many produce new root systems at intervals along the lateral stem: these are the rhizomatous and stoloniferous plants. The connections between the parts of such plants may die and rot away, so that the product of the original zygote becomes represented by physiologically separated parts. (Modules with the potential for separate existence are known as ‘ramets’.) The most extreme examples of plants ‘falling to pieces’ as they grow are the many species of floating aquatics like duckweeds (Lemna) and the water hyacinth (Eichhornia). Whole ponds, lakes or rivers may be filled with the separate and independent parts produced by a single zygote.

Trees are the supreme example of plants whose growth is concentrated vertically. The peculiar feature distinguishing trees and shrubs from most herbs is the connecting system linking modules together and connecting them to the root system. This does not rot away, but thickens with wood, conferring perenniality. Most of the structure of such a woody tree is dead, with a thin layer of living material lying immediately below the bark. The living layer, however, continually regenerates new tissue, and adds further layers of dead material to the trunk of the tree. This solves, by the strength it provides, the difficult problem of obtaining water and nutrients below the ground, but also light, perhaps 50 m away at the top of the canopy.

modules within modules

We can often recognise two or more levels of modular construction. The strawberry (Figure 4.1c) is a good example of this: leaves are repeatedly developed from a bud, but these leaves are arranged into rosettes. The strawberry plant grows (i) by adding new leaves to a rosette and (ii) by producing new rosettes on stolons grown from the axils of its rosette leaves. Trees also exhibit modularity at several levels: the leaf with its axillary bud, the whole shoot on which the leaves are arranged, and the whole branch systems that repeat a characteristic pattern of shoots.

Many animals, despite variations in their precise method of growth and reproduction, are as ‘modular’ as any plant. And in corals, for example, just like many plants, the individual may exist as a physiologically integrated whole, or may be split into a number of colonies – all part of one individual, but physiologically independent (Hughes et al., 1992).