BOT 360F - Families of Vascular Plants

Lecture notes,

Gymnosperms

"plants with naked ovules"


Heterospory and seeds

...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) .
...archegonia 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).
...meanwhile, 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 sperm cells.
...pollination 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 1975). The earliest seeds appeared in the Late Devonian. Click HERE to visit a site with links to illustrations of a great many of these earliest seeds.
Ginkgo biloba ovule and seed development.

Ginkgo seed development

Note 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.


| What are plant families? | How do we distinguish them? | How and why do we study them? | Selected vascular plant families of Ontario | Reading List | Course outline |

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