In almost all modular organisms (see Section 4.2.1), an individual genet branches and spreads its parts around it as it grows. There is a sense, therefore, in which a developing tree or coral actively disperses its modules into, and explores, the surrounding environment. The interconnections of such a clone often decay, so that it becomes represented by a number of dispersed parts. This may result ultimately in the product of one zygote being represented by a clone of great age that is spread over great distances. Some clones of the rhizomatous bracken fern (Pteridium aquilinum) were estimated to be more than 1400 years old and one extended over an area of nearly 14 ha (Oinonen, 1967).
We can recognize two extremes in guerrillas and a continuum of strategies in clonal dis phalanx-formers persal (Lovett Doust & Lovett Doust,
1982; Sackville Hamilton et al., 1987). At one extreme, the connections between modules are long and the modules themselves are widely spaced. These have been called 'guerrilla' forms, because they give the plant, hydroid or coral a character like that of a guerrilla army. Fugitive and opportunist, they are constantly on the move, disappearing from some territories and penetrating into others. At the other extreme are 'phalanx' forms, named by analogy with the phalanxes of a Roman army, tightly packed with their shields held around them. Here, the connections are short and the modules are tightly packed, and the organisms expand their clones slowly, retain their original site occupancy for long periods, and neither penetrate readily amongst neighboring plants nor are easily penetrated by them.
Even amongst trees, it is easy to see that the way in which the buds are placed gives them a guerrilla or a phalanx type of growth form. The dense packing of shoot modules in species like cypresses (Cupressus) produces a relatively undispersed and impenetrable phalanx canopy, whilst many loose-structured, broad-leaved trees (Acacia, Betula) can be seen as guerrilla canopies, bearing buds that are widely dispersed and shoots that interweave with the buds and branches of neighbors. The twining or clambering lianas in a forest are guerrilla growth forms par excellence, dispersing their foliage and buds over immense distances, both vertically and laterally.
The way in which modular organisms disperse and display their modules affects the ways in which they interact with their neighbors. Those with a guerrilla form will continually meet and compete with other species and other genets of their own kind. With a phalanx structure, however, most meetings will be between modules of a single genet. For a tussock grass or a cypress tree, competition must occur very largely between parts of itself.
Clonal growth is most effective as a means of dispersal in aquatic environments. Many aquatic plants fragment easily, and the parts of a single clone become independently dispersed because they are not dependent on the presence of roots to maintain their water relations. The major aquatic weed problems of the world are caused by plants that multiply as clones and fragment and fall to pieces as they grow: duckweeds (Lemna spp.), the water hyacinth (Eichhornia crassipes), Canadian pond weed (Elodea Canadensis) and the water fern Salvinia.
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