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Plants need not produce seed at all. One of the remarkable features of plants is their capacity for clonal growth – growth without sexual reproduction. The bodies of mammals reach a certain size and stop growing; then they require sexual activity for reproduction. Most plants, in contrast, have multiple growing points called meristems, and if some meristems are damaged, the others will continue growth.

Plants can therefore grow to indeterminately large sizes – either in height, as in trees, or laterally, as in many herbaceous plants.

Clonal reproduction appears to be particularly common in stress-ful habitats (Grime 1977) such as deserts (Gibson and Nobel1986), tundra (Savile 1972), freshwater wetlands (Sculthorpe1967) and salt

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(b) Figure 4:6 A selection of

dust-sized seeds found in the British flora. (a) Gymnadenia conpsea (an orchid), (b) Orobanche elatior (parasitic), (c) Pyrola secunda, (d) Pyrola media, (e) Drosera anglica, (f) Crassula tillaea, (g) Digitalis purpurea, (h) Polycarpon tetraphyllum, (i) Narthecium ossifragum, (j) Scrophularia nodosa, (k) Spergularia rubra, (l) Samolus valerandi (from Salisbury1942).

marshes (Adam 1990). In these habitats, the aerial shoots of each clone are interconnected by a dense interwoven mass of rhizomes (Figure4.7). Clones can be extensive. In the common bracken fern (Pteridium aquilinum), clones may exceed 1000 m in diameter (Parks and Werth1993). The world’s largest plant (by mass) is a single clone of aspen (Populus tremuloides) with 47 000 stems connected below-ground, and covering more than 42 ha (Grant1993).

Why do plants have this clonal capacity? Stress may limit the resources needed to construct flowers, it may reduce the availa-bility of pollinators, but most often it likely reduces the probaavaila-bility of regeneration from seed. There is a clear evolutionary trade-off: produce large numbers of small seeds, each with a low proba-bility of establishment, or allocate the same resources to fewer larger clonal offspring, each having a higher probability of establishment.

Considerable effort has gone into unravelling the advantages and disadvantages of clonal growth (e.g., Jackson et al.1985, Harper et al.

1986), but three older models (Williams1975) still retain their value.

The names used for Williams’s models include animals, and I have not tried to rename them in strictly botanical terms, although you may find this a useful, if challenging, exercise. Here are his three models that address clonal growth, sexual reproduction, and environ-mental conditions.

Figure 4:7 The arrangement of aerial shoots and rhizomes in an established sward of Calamagrostis neglecta in Iceland (from Kershaw 1962).

1. The Aphid-rotifer model

The model name refers to the life cycle of aphids and rotifers. Both cycle between periods of asexual reproduction in the summer and periods of sexual reproduction toward the autumn. Clonal reproduc-tion may be advantageous during periods with relatively stable envir-onmental conditions, whereas sexual reproduction may be reserved for times when the environment begins to change. Future environ-mental conditions that will arise from the change are unknown.

Sexual reproduction, which results in greater variation among off-spring, increases the likelihood that some will be better adapted to surviving under the new conditions. Continued clonal reproduction would produce only individuals adapted to the environment before the change occurred.

2. The strawberry-coral model

Strawberries and corals both produce colonies by clonal reproduc-tion, but can also produce large numbers of small, sexual offspring.

Clonal reproduction may be advantageous within relatively desirable patches of habitat, whereas sexual reproduction may be reserved largely for dispersal among patches.

When a young strawberry seedling first becomes established, it develops stolons that asexually produce new individuals. This pro-cess could continue without limit – as long as the favorable patch is very large. Yet the more the plant expands, the greater the number of shoots that will arise ever further from the location where the plant first established as a seedling. Hence, as the plant expands, the odds increase that a shoot will encounter less favorable conditions. At some point, further allocation of resources to new shoots at the edge of the clone will not result in more offspring. At this point, sexual repro-duction allows dispersal to new sites.

Occasional sexual reproduction is also advantageous when patch characteristics change as described for the previous model. No patch will survive forever. The change may be physical, such as a drought or fire that erases the patch and replaces it with a different set of conditions. The change may also be biological, since new genotypes and species are continually dispersing seeds into the patch. The big-ger the clone, the more reproductive propagules it can produce, and so there may be a selective advantage to those clones that delay sexual reproduction until the local patch is filled. The strawberry-coral model is similar to the aphid-rotifer model, but it address the trade-offs created by environmental conditions that change in space rather than in time.

3. The elm-oyster model

In some cases, clonal reproduction is unlikely to be advantageous and there may be strong selection for sexual reproduction. The model name refers to those species, like elm trees and oysters, that can be thought of as having no clonal reproduction – single individuals hold one small piece of space and flood adjoining areas with sexually

produced offspring. Elms produce vast numbers of seeds, and it should be readily apparent that one elm tree can produce vastly more seedlings than can possibly establish as adult trees. The space occupied by each adult elm tree is not unlike the single patch occu-pied by the clone of a strawberry. Large numbers of seedlings may colonize a patch, but owing to intense competition among them, a site will tend to be occupied by the one individual that is marginally better at exploiting resources, suppressing neighbors, and minimiz-ing losses to pathogens and herbivores. Over evolutionary time, those trees that survived were not those most successful at making more copies of the same genotype, but rather those producing offspring with many different genotypes with differing competitive abilities.

In addition to the potential importance of clonal growth as a repro-ductive strategy discussed above, it has other advantages, particularly in areas where water or nutrients are in very low supply. The inter-woven webs of rhizomes (Figure 4.7) may permit shoots to share resources. Shoots that are located in stressful patches may be able to use resources transferred from adjoining shoots in richer patches. The branching pattern of a plant may reflect its style of foraging in an array of such patches (Bell1984, Huber et al.1999), and we have already seen an example of grasses foraging in patches of nitrogen (Section3.5.3).

This proposition can be experimentally tested. For example, Salzman and Parker (1985) experimentally evaluated the physiologi-cal connection among shoots of the herbaceous perennial Ambrosia psilostachya. Pairs of shoots connected by a section of rhizome were grown in partitioned pots that were watered with either freshwater, saltwater (the stress condition), or a split environment with half of each. Shoots in the saline treatments produced only 34 percent of the biomass of those in the freshwater treatments, illustrating the neg-ative effects of salinity on plant growth. If, however, shoots in saline soil were connected to shoots in fresh water treatments, they then reached 74 percent of the biomass of fresh water treatments: ‘‘thus, connection to a stem in a non-saline soil greatly ameliorated the harmful effects of salt.’’ Moreover, they add: ‘‘rhizome connections among plants in contrasting environments do not result in a simple averaging of the effects of the two environments; the benefits received by plants in a stressful environment outweighed the costs incurred by their connected neighbors in a favorable environment.’’

A similarly designed experiment (Slade and Hutchings1987) used another herbaceous perennial, Glechoma hederacea, in three soil nutrient regimes: nutrient rich, nutrient poor (the stress), and split treatments. Although growth was more vigorous in the fertile treat-ments, the results for split treatments were intermediate between the pure treatments. Hence the conclusion that there was ‘‘no integ-ration between primary stolons subjected to different nutrient levels.’’ Glechoma hederacea is, however, a common species in woods, grasslands, and disturbed sites; it is not normally found in sites that would be called stressed so much as disturbed. The ability to endure