Introduction
Gymnosperms are an informal grouping of vascular seed plants with naked seeds (no fruits), that with angiosperms comprise the "seed plants"
they have seeds (= embryo + food supply + seed coat)
they can produce wood
Wood
Primary growth vs. secondary growth
lateral meristems (vascular cambium)
woody vs. herbaceous
vascular cambium
in gymnosperms it is:
- bifacial
- capable of unlimited growth
it produces:
secondary xylem -- tracheids
secondary phloem -- sieve cells
also rays, which allow radial transport in xylem and phloem
cork cambium
produces cork cells of outer bark
inner bark is phloem
a little Phylogeny
the megaphyll line gave rise to the progymnosperms
the progymnosperms gave rise to the aneurophytes and archeopterids
aneurophytes -- protostele, homosporous, simple photosynthetic structures
archeopterids -- eustele, heterosporous, evolved megaphylls
the archeopterids gave rise to the various extant gymnosperm divisions:
Ovule and Seed
Ovule = megaspore (initially a megasporocyte) + megasporangium (= nucellus) + integuement
Compare and contrast with life cycle of Selaginella
- both are heterosporous
- megagametophyte (= nucellus) is reduced and retained on sporophyte
- megasporangium is surrounded by an integument
- ovule = megasporocyte (which gives rise to a megaspore) + nucellus + surrounding integument
Ovule becomes the seed
Seed = embryo + food + seed coat
click here to go to introduction to gymnosperm vocabulary
on to the next topic -- gymnosperms!
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