BOT 360F - Families of Vascular Plants 

Reproductive Morphology (1)


Supplementary notes for lecture on 25 September 2012

 

 

The life cycle diagrammed below is common to all land plants, both free-sporing ones and seed plants. Land plants are also called Embryophytes because their life cycle includes a stage in which the immature sporophyte is retained within the gametophyte (the extreme example of this is seen in the Bryophytes, where even the mature sporophyte is mechanically and nutritionally dependent on the gametophyte).

Free-sporing plants

  • In land plants, meiosis in portions of the diploid sporophyte (the sporangia) gives rise to haploid spores (i.e. land plants exhibit sporic meiosis).
  • In the free-sporing plants spores are liberated into the air, and dispersed, so as to produce new plants, in new locations, when the spores germinate.
  • Spores often have elaborately thickened, sporopollenin-encrusted walls that protect the cytoplasm within from dessication and other injury.
  • Spores germinate and give rise to the (more or less) free-living, non-vascular, gamete-producing (gametophyte) stage of the life cycle. [Note that although vascular tissue is lacking in the gametophytes of free-sporing vascular plants, vascular tissue can nevertheless be induced to form by the application of sucrose, and of various plant hoermones (Demaggio, A. E. 1972, Bot. Gaz. 133(3): 311-317).]
Fern spore with trilete ridge

Spore of the fern, Pityrogramma austroamericana. Scale bar is 10µm. Click on image for more information.

Copyright © 2010 Robbin Moran

Seed plants

  • In seed plants (and in a few free-sporing ones) the spores, hence the sporangia, are of two types: microsporangia within which meioses result in the formation of (many) small microspores, and megasporangia within which meiosis results in the formation of (one to a few) much larger megaspores. This is called heterospory; the contrasting conditions (only one kind of spore) is homospory.
  • Pollen grains are endosporic microgametophytes, i.e. gametophytes that develop within the microspore wall as a result of as few as one to 10 or more mitotic divisions. The final divisions result in the formation of a pair of sperm nuclei.
  • Ovules are more complicated: at maturity they contain endosporic megagametophytes, enclosed not only by the megasporangium (the nucellus) but also additional sporophyte tissue (the integuments).
  • Endosporic megagametophytes develop from one (or more) of the megaspores.
  • Endosporic megagametophytes at maturity comprise an egg cell together with varying numbers of other cells that are not, for the most part, directly involved in reproduction. Fertilization of the egg following pollen capture and introduction of sperm nuclei to the megagametophyte produces a zygote that develops into an embryo.
  • Seeds are matured ovules, within which the embryo has completed development and typically is in a dormant state. In addition to the embryo sporophyte, seeds contain nutritive tissue (that may have become incorporated into embryo), and are enclosed by a seed coat (the integuments).

In summary, in comparison with the free-sporing plants, seed plants have encapsulated their gametophytes in sporophytic tissue so as to

  • make the process of sexual reproduction much, much less dependent on the availability of liquid water (microgametophytes are delivered into close proximity to a megagametophyte);
  • make the process of sexual reproduction much more efficient, since sporophytic structures can be elaborated in order to employ wind, water, or animals to assist with microgametophyte delivery;
  • and make the process of dispersal much more likely to succeed by dispersing multicellular embryos rather than unicellular spores.

As a result, reproduction in seed plants may appear to result in a succession of sporophyte generations when in fact the same alternation of gamete- and spore-producing generations seen in the free-sporing plants continues to occur.

  land plant life cycle
 

 

 


Floral morphology (cf. trip to Wards Island)

At Wards Island we had a chance to look at a number of of flowers (Oenothera, Helianthus, Symphyotrichum, Trifolium, etc.). All of these conformed to the basic plan of organization shown below, variously modified as shown on the flowering plant reproductive morphology page.

Magnolia flower - diagrammatic - Compare with the photograph at right.

Photo © 1999 R. C. Evans

floral morphology Magnolia flower

Oenothera biennis - common evening primrose. Note the inferior ovary (and the fruits that develop from it), as well as the long hypanthial tube. You may recall that the flower is 4-merous, that is, the floral parts are in multiples of four (four free calyx lobes, four petals, eight stamens, a 4-chambered ovary, and a 4-lobed stigma).

Photo © 2004 Royal Ontario Museum (ROM The Field Guide to Wildflowers of Ontario, p. 313)

Oenothera biennis WOO  



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