The Elements of Evolution (51-3-1) Seeds

 Seeds

Seed survival, dispersal and persistence in soil determine the composition and dynamics of plant communities. ~ English botanist Louise Colville et al

For spermatophytes (gymnosperms and angiosperms), seeds start the engine of life. Tiny annual plants may produce 10 seeds within a few weeks before dormancy, while giant conifers produce huge seed crops over centuries. For a plant investing in the next generation, a trade-off exists between number and size.

Bigger seeds beget more competitive seedlings, but they cost more to produce, have a lower dispersal distance, and a higher risk of predation. (Exceptionally, coconuts managed worldwide dispersal despite huge seeds. Their seed strategy was uniquely far-sighted. But then, one’s perspective is different when life’s a beach.) Larger seeds tend to germinate quickly, but this growth spurt advantage leaves a young plant less likely to prosper in the face of intense herbivory.

Producing enormous numbers of seeds is common for plants that have grown in good conditions. A single poppy plant may produce a million seeds. Yet few find a suitable location for germination, and most will be eaten by a ravenous horde of herbivores.

The alternative life-history is for a seed is to be swaddled in protection; its mother trading raw numbers for better odds. So it is with the coconut. Only a smattering of coconuts are made by an individual tree. A strong fibrous cover and hard shell protects the embryo against most herbivores. The milk within is a life-giving care package, consumed when a suitable home is found. A coconut typically starts life when its nut washes ashore, but what signals the embryo to initiate germination is unknown.

As illustrated with coconuts, coating is a life-history variable for seeds. Animal seed eaters naturally prefer soft seeds, whose consumption spells an end to their germination potential. Soft seeds are more easily located via their cocktail of volatile compounds that give their presence away. By contrast, hard seeds, even if detected, may survive a 2nd dispersal event, outlasting digestion or the travails of hoarding by a fastidious seed eater.

Most hard-seeded plants produce dimorphic seeds. In terms of reward, dimorphic seeds support the hypothesis that the primary evolutionary explanation for hard seeds is predator escape. ~ Norwegian evolutionary ecologist Torbjørn Paulsen

Seed dispersal may be had by the wind, water, or animal agent. Presentation and packaging of seeds depends upon the path to propagation.

Environmental uncertainty leads plants to adopt a mixed strategy for seed dispersal. Annuals often create seeds in different packages: some designed for long distance travel while others drop nearby. Potential competition between siblings is instrumental in this decision-making. Plants in the sunflower family are exemplary: fruits of the outermost flowers lack parachutes, while the more numerous, smaller inner seeds are readily wind-dispersed via pappi.

For annuals, the advantages of fecundity and survival via dormancy are offset by size constraints and the need for fertile soil. The disadvantages are offset by a hedging strategy: seeds that germinate under different conditions.

Synchronous sprouting of an entire seed crop would risk total loss if environmental conditions deteriorated. Thus, individual plants give any single clutch of seeds a range of dormancy-breaking and germination thresholds.

For a seed coat to split, the embryo must imbibe (soak up water), causing the seed to swell and split. Only then can germination begin.

Seed embryos monitor their situation and may resolve upon a false start. A seed may imbibe, but still decide that conditions are insufficient to risk germination – so, the embryo waits.

There are 2 forms of seed dormancy: physiological and physical. Soft seeds rely solely on physiological dormancy: sprouting when sensing an opportune time. Hard-shell seeds also have physical dormancy. Hard-seededness helps protect against microbial attack, reduces external chemical signaling of presence, and extends seed longevity, at the cost of a plant producing a hard coat for each of its latent offspring.

Many plant species rely on hoarding rodents for secondary seed dispersal. Hard seeds are surprisingly frequent in hot deserts, where granivory by small rodents is very intense. ~ Torbjørn Paulsen et al

Hard seeds are used by only a few plant families. Legumes are one of them. At least 6 times, different legumes independently decided to pay extra for hard coating, which both reduces olfactory signaling and improves survivability when a seed is snatched.