"plants with naked ovules"
- ...the megasporangium
(nucellus) develops within an integument, an additional,
enclosing sporophytic structure. Within the nucellus a single megaspore
mother cell undergoes meiosis to form four megaspores.
- ...only one
of these megaspores is functional, undergoing repeated nuclear divisions
so as to give rise to the female prothallus (megagametophyte) .
develop after cell walls are laid down. At the same time, the integument
matures, typically with formation of an inner, stony layer (sclerotesta)
and what will become an outer, fleshy layer (sarcotesta).
pollen grains are released from the microsporangia. These consist
of the male prothallus (microgametophyte) that develops within the wall
of the microspore. Male prothalli may comprise from three to tens of
cells by the time they are mature, at least two of which are functional
involves capture of pollen grains by the ovule. The pollen grains
germinate, forming a pollen tube that delivers sperm cells to the vicinity
of the archegonia. Discharge of the sperm leads to fertilization and
development of the embryo sporophyte.
- ...the seed
develops as a result of the coordinated growth of the embryo and
the surrounding sporophyte tissues (integuments and other structures
associated with the ovule), growth of the former occurring at
the expense of the megagametophye.
- ...in Ephedra
and Gnetum two zygotes are formed, only one of which apparently
develops into the definitive embryo (Friedman
& Carmichael 1996).
- ...extinct heterosporous
pteridophytes appear to have given rise to the seed plants (Sporne
earliest seeds appeared in the Late Devonian. Click HERE
to visit a site with links to illustrations of a great many of these
biloba ovule and seed development.
that in Toronto the stage shown at the bottom of this diagram occurs at about
the same time as this lab, i.e. at the end of September.
Immature and mature seeds (above and right, respectively) showing embryo in relation to megagametophyte and tissue of the parent sporophyte. Note the enlarged, fleshy outer portion of the integument (sarcotesta) and the inner, hard portion shown in black (sclerotesta), in the mature seed.
These diagrams adapted from Fig. 22-15 in Bold, H. C. et al. (1987), Morphology of plants and fungi, 5th ed. New York, Harper & Row.
are plant families? | How do we distinguish
them? | How and why do we study them? |
Selected vascular plant families of Ontario
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2008 Department of Ecology & Evolutionary Biology and 2000-2006 Botany Department, University of Toronto.