seeds and early development Flashcards
soil seed bank
all viable seeds found in the soil, duration of viability is variable anywhere from 5 to 2000 years
seed composition
seed coat made of maternal tissue surrounding an embryo and a food source
endosperm
food source found in the seeds of angiosperms, can be persistent or absorbed into the cotyledons which then become fleshy
embryo
all contain a radicle (embryonic root), cotyledons (embryonic leaves), and a hypocotyl (embryo below cotyledons
some contain an epicotyl (embryo above cotyledons), and a plumule (tip of the epicotyl including true leaves and the apical meristem)
funiculus
stalk that connects seeds to the ovary like an ambilocal cord
hilum
scar on a seed coat where the funiculus was previously attached
micropyle
small hole at the end of the hilum on the seed coat, functions in sperm entry to the ovule, water entry during development, and the weak point for radicle emergence
advantages of seeds
they can wait to grow until the environment is more favorable, protect the embryo from harsh environments, allow the embryo to travel
seed size
highly variable, smallest in orchids which have no endosperm and require a fungal partner to germinate, largest is the double coconut at 37lbs
stages of embryo development
embryo growth and differentiation, seed expansion and endosperm accumulation, desiccation, quiescence, imbibition, germination
desiccation
completely drying out of the seed to prevent rot
quiescence
metabolically inactive state of seeds
imbibition
rehydration that resumes metabolism
germination
resumption of growth by a quiescent or dormant seed requiring at least, water, oxygen, and a minimum temperature
dormancy
longer period of metabolic inactivity, that requires some extra step to germinate such as scarification, intense heat, stratification, alternating periods of cold and warm, chemical removal
scarification
physical breakdown of the seed coat from a variety of sources such as animals digestion or repeated freeze thaw cycles
intense heat
requirement to overcome dormancy that reduces the number of competitors and increases the nutrients in the soil
stratification
requirement to overcome dormancy, extended period of cold
seed coat dormancy
seed coat physically prevents water entry so germination cannot occur until it is removed
embryonic dormancy
the embryo itself requires the required stimulus to germinate
morphological dormancy
when the seed is fully developed the embryo is still in the globular stage
photodormancy
embryo needs light
thermal dormancy
embryo requires specific temperature conditions
chemical dormancy
a chemical must be removed from the embryo
viviparity
seed germinates while still in the fruit on the parent plant
recalcitrant seeds
never fully desiccate, germinate as soon as the minimum requirements are met
when germination occurs
seed imbibes water, embryonic root and shoot grow, radicle emerges, stem emerges, true leaves expand, there is a switch from stored nutrients to photosynthesis
epigeous growth
cotyledons emerge above ground, hypocotyl forms a hook dragging cotyledons up, once above ground hook straightens and epicotyl grows
hypogeous growth
cotyledons remain below ground and inside the seed coat, epicotyl forms a hook, emerges, straightens out, and the true leaves unfold
elaiosome
fatty body on a seed attracts ants to carry seeds away
non mutualistic seed dispersal
mechanisms of sticking to animals like burrs, ballistic, wind, or water dispersal
