root-shoot axis - regulating amount/pattern of cell division >> 3D shape/form
- cells w/ multiple potentials restricted to meristemic regions
- apical meristems establish axis in embryogenesis
- tissue systems organized radially around axis
- food source for embryo - endosperm in angiosperms, megagametophyte in gymnosperms
- 1st division of zygote >> smaller cell becomes embryo, larger cell forms suspensor (links embryo to nutrient tissue in seed)
- cells near suspensor >> root
- cells at other end >> shoot
- embryo stops suspensor from developing into another embryo
tissue systems - dermal tissue, ground tissue, vascular tissue
- dermal tissue - from protoderm, outermost cells of embryo
- divides w/ cell plate perpendicular to surface
- ground tissue - functions in food/water storage
- vascular tissue - from procambium at embryo core
- functions in water/nutrient transport
- formation of roots/shoots controlled independently
- morphogenesis - generation of form, produces the cotyledon(s)
- seeds need enough nutrients to support sporophyte until it can photosynthesize
- can’t bury seeds to deep
germination - radicle (1st root) extends through seed coat
- orients so that roots grow down, shoots grow up
- starts when seed absorbs water, metabolism resumes
- most seeds must stratify (spend time in cold conditions) before germinating
- ensures that seed will not sprout right before cold season
- amyloplasts - starch-storing plastids, provides metabolic reserves for young plant
- cotyledon >> scutellum (food source used before endosperm)
- aleurone - outer layer of endosperm, signaled by gibberellic acid to produce amylase
- seed very susceptible to disease/drought between germination and young plant stage
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