Erica's Original Notes The invention of an encased seed was more important than that of the wheel. Nature's packaging of seeds outshines man's efforts at packaging. Each seed carries its own backpack of food with a precise formula of food until the baby plant can start feeding through its own roots. One is amazed at the power of a small seed to build a tree out of air and water; this infinitesimally small unit can raise a forest of trees or a garden of flowers.
Seed parts: Each seed is made up of three parts: a) the coat for protection; b) the food supply (albumin, starch and oil); and c) embryo or new baby plant.
Hilum: scar left from the stalk attachment in flowering plants seeds.
Gymnosperms: the 'naked' seeds of the conifers. 'Gymno' comes from the Greek word and is used in our word `gymnasium', because the Greeks attended this exercise arena naked.
Parthenocarpy: where the fruit develops without fertilization and this means there are no seeds, as in the banana and Conference pear.
Hormone inhibitors: found in seeds, especially important in early seed-setting annuals, to prevent them from germinating before the fall rains.
Weakening of seed coat: may occur by variation of temperature, moisture, bacteria and fungus (lotus seeds contain an antibiotic to delay this process); animal and bird guts help break down the seed coat. (Galapagos tomato will only germinate if eaten by a Galapagos tortoise!).
Scorching by fire breaks down seed coat in some species such as Acacia, Epilobium and Pinus contorta Climatic vernalization, meaning cold winter conditions, allows chemical inhibitors in the seed coat to be destroyed. Seeds can also be abraded by sulphur and nitric acid and aided by human intervention. A great deal of parental care is lavished on getting the seed prepared for its journey, and, hopefully, new life, and there are many aids to help distance the new plant from its parent.
Seed planting aids:(See note 1 below) Seed adaptations: a) Floating seeds in water are protected by a wax coat. b) Mucilage exuded by some seeds when wet helps to stabilize them in the soil. c) Seed of Dicentra (Pacific bleeding heart) has a sac of gelatinous fat (elaiosome) attached as a 'come-on'. Ants carry the fat and the seed home, the ants get the food, and the seed gets transported to a new place to grow.
Fruit adaptations: a) Seeds embedded in edible tissue, such as arils of the yew, attract eaters. b) Barbed devices enable fruit with seeds to get attached to a passing animal or human, who will then carry them off to a different location. c) Hydroscopic devices such as are found in the resurrection plant, which rolls up when dry but opens with moisture and ejects its seeds. Cress has horizontal pods which open when hit by raindrops. d) Shakers, e.g. poppies and larkspurs. e) Explosive devices caused by tension created in the drying pod will then rupture; e.g. broom, cranesbill. f) Birds also get seeds in their feathers. It is interesting to note that seeds they eat are evacuated after only a half hour. g) Oils attract carnivores as well as vegetarians (olives by wild dogs and cats; avocados by jaguar), not that we need to worry about this here. This is a link with the past when reptiles ate and spread the seeds of cycads and ginkgo. h) Ant distribution: In Cyclamen, several mechanisms assist dispersal of the seeds. The flower stem coils or bends towards the ground when the fruit begins to form. The fruit, which is a round pod, opens at maturity to release numerous sticky seeds, which are then dispersed by ants, which eat the sticky covering and then discard the seeds.
In the section on Seed planting aids, I have changed the order of Erica’s notes to separate seed adaptations and fruit adaptations. In flowering plants (Angiosperms) the seed is not naked and is borne in a variety of fruits. Many fruits have mechanisms that assist in the dispersal of their seeds and Erica cites many of these.