#39 - Apr 2020 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
SEEDSApril/May: Mid/late autumn. Cooling down. Bubble living’s made some think about growing veggies from seed.
Seed plants: These appeared ~350 m years ago. Two sorts: gymnosperms ‘exposed seeds’ (in a cone) and later angiosperms, ‘covered seeds’ (in a fruit).
Angiosperm seeds: Après pollination, the flower changes. Petals fall off. The now-fertilised egg cell morphs to an embryo, the surrounding ovule to a seed, and the surrounding ovary to a fruit.
Soon the embryo stops growing. The now-mature seed contains a tiny dormant embryo, an endosperm (food reserves) and is surrounded by a seed coat. The surrounding fruit continues to develop.
Later, the fruit ripens, falls to the ground and rots, or is eaten. The embryo in the seed stays dormant. The seed may pass through the gut of an animal.
It waits for the right moment to wake. It can wait a long time. In 73 AD, King Herod spat out a date stone in Masada. It germinated in 2003.
Dispersal: It’s not good if a seed germinates near its parent. Better to try elsewhere. Seeds hitchhike. Buoyant coconuts (in the sea), dandelion fairies (in the wind), grape seeds (in birds), prickly grass seeds (caught in fur). Coevolution of angiosperms and animals…
Size matters: There’re two opposite seeding strategies - a few big seeds with a high chance of survival (coconut, 1.5 kg) or very many small seeds with a low chance of survival (poppy, 0.3 mg). The best strategy is the one that maximises the chances of producing a mature offspring, in the right habitat.
Dormancy: Seeds of most species lie dormant for years before germinating. Why? The dormant seeds form a 'seed bank' in the soil. Just a few germinate each year. Why is this good...?
If this year’s seedling cohort all perish (drought, fire, predators) - no matter… Next year’s cohort will germinate just the same.
If all seeds germinated at once, the species could be wiped out by one disaster.
Germination: First, the seed absorbs water. This wakes its dormant metabolism. After its long lockdown, the embryo restarts growth, using up the endosperm reserves. The embryo first grows a root (down) to ensure access to water, it then grows a shoot (up) to ensure a continuing food supply (photosynthesis). Once a dormant seed’s been woken, it’s very vulnerable to drying (almost no root). Keep seeds moist the first days/weeks after sowing or they die. ___________________________________
SEEDSApril/May: Mid/late autumn. Cooling down. Bubble living’s made some think about growing veggies from seed.
Seed plants: These appeared ~350 m years ago. Two sorts: gymnosperms ‘exposed seeds’ (in a cone) and later angiosperms, ‘covered seeds’ (in a fruit).
Angiosperm seeds: Après pollination, the flower changes. Petals fall off. The now-fertilised egg cell morphs to an embryo, the surrounding ovule to a seed, and the surrounding ovary to a fruit.
Soon the embryo stops growing. The now-mature seed contains a tiny dormant embryo, an endosperm (food reserves) and is surrounded by a seed coat. The surrounding fruit continues to develop.
Later, the fruit ripens, falls to the ground and rots, or is eaten. The embryo in the seed stays dormant. The seed may pass through the gut of an animal.
It waits for the right moment to wake. It can wait a long time. In 73 AD, King Herod spat out a date stone in Masada. It germinated in 2003.
Dispersal: It’s not good if a seed germinates near its parent. Better to try elsewhere. Seeds hitchhike. Buoyant coconuts (in the sea), dandelion fairies (in the wind), grape seeds (in birds), prickly grass seeds (caught in fur). Coevolution of angiosperms and animals…
Size matters: There’re two opposite seeding strategies - a few big seeds with a high chance of survival (coconut, 1.5 kg) or very many small seeds with a low chance of survival (poppy, 0.3 mg). The best strategy is the one that maximises the chances of producing a mature offspring, in the right habitat.
Dormancy: Seeds of most species lie dormant for years before germinating. Why? The dormant seeds form a 'seed bank' in the soil. Just a few germinate each year. Why is this good...?
If this year’s seedling cohort all perish (drought, fire, predators) - no matter… Next year’s cohort will germinate just the same.
If all seeds germinated at once, the species could be wiped out by one disaster.
Germination: First, the seed absorbs water. This wakes its dormant metabolism. After its long lockdown, the embryo restarts growth, using up the endosperm reserves. The embryo first grows a root (down) to ensure access to water, it then grows a shoot (up) to ensure a continuing food supply (photosynthesis). Once a dormant seed’s been woken, it’s very vulnerable to drying (almost no root). Keep seeds moist the first days/weeks after sowing or they die. ___________________________________