An Online Introduction to the Biology of Animals and Plants

Key Concepts

Section 2

Chapter 2

Primitive Land Plants

BRYOPHYTES

Two major types of plants eventually colonized the land:  the bryophytes (the suffix -phyte is from a Greek word for "plant") and the vascular plants.  Bryophytes include moss, a primitive form called liverworts, and an even lesser-known form called hornworts.  These and the lichens were probably the original pioneer organisms on land, and still are often the first plants to take hold in inhospitable places.

Bryophytes developed support structures and photosynthetic surfaces, and sometimes look like vascular plants, but they lack a trait that has made the vascular plants much more successful:  a tube system used to move water beneath the ground up to the above-the-ground photosynthetic structures, and to move fuel down to the the buried water-gathering structures (bryophytes are called nonvascular plants).  Without this type of vascular system, there are major limits on how tall a plant can be.  Bryophytes depend upon either environmental water from the air (like dew) or the limited upward movement water can take against gravity by either diffusing through the plants (moving just by spreading from a high-dilution area to a low-dilution area) or by adhesion, movement from the "stickiness" that water has for molecules like the starches plants use for structure (almost like how water spreads through a paper towel).

When the challenges of living on land were discussed in Section 1, Chapter 7, it was mentioned that the toughest problem to solve concerned sexual reproduction that used swimming sperm, and this was true for plants:  long after the drying, support, and other problems were adapted to, the reproduction problem existed.  As also happened in the first vascular plants, bryophytes developed an alternation of generations pattern, with a sexual phase, the gametophyte, alternating with an asexual phase, the sporophyte.  Unlike the vascular plants, whose "main" (most obvious) form is the sporophyte, what we think of as "moss" is the gametophyte form.  A moss sporophyte is a small attached form that makes, not surprisingly, spores.

LIFE CYCLE OF A TYPICAL MOSS

Moss spores, containing only one set of chromosomes (haploid), are carried by the wind from the parent sporophyte.  If they land somewhere with the right moisture content (bryophytes don't do well if it's too dry), the spore begins to divide and produce very algae-like filaments that cover the surface it's on.  From that surface will sprout the gametophyte, with structures to hold it to the surface called rhizoids (rhiz- is from a word for "root") and upward-growing "stems" with "leaves" that are only a single cell layer thick.  (Although parts of a moss may resemble roots, stems, and leaves, they are not truly these structures because the "real things" always have vascular structures inside them - that's why they are being called "stems" in quotes, etc.)  The materials needed by the moss to grow are absorbed by the upper structures rather than the rhizoids, but this allows mosses to grow on fairly sparse surfaces.

Some of the moss gametophytes are male and some are female (see below for an explanation of the differences), and the very tips form sexual reproductive structures called antheridia (male) and archegonia (female).  The antheridia swell up and burst, and when the plants are wet with rain or heavy dew, release sperm that swim through the water coating the plants to the archegonia, where they fertilize waiting egg cells.  The fertilized egg, now a zygote containing two sets of chromosomes (diploid), grows into a small sporophyte.  Spores are eventually produced by a type of cell division called meiosis, creating haploid spores from diploid sporophyte cells.  The tip of the sporophyte opens, releasing the spores into the winds, where they can travel great distances and still sprout even decades later.

HOW CAN A MOSS PLANT BE MALE OR FEMALE?

Back in Section 1, Chapter 6, in the discussion about sexual reproduction, it was mentioned that such reproduction does not require two separate parents or even male and female sources, although different genders are extremely common.  As we've just discussed, mosses can be male and female;  so can flower parts, pine cones, parts of a flatworm, and jellyfish.

You may think you know the basics of telling a male from a female, but you can't peek under a moss looking for noogies and a penis.  What makes a form male?

Gender is determined by the type of gamete produced.  A gamete is a sex cell - sperm is the male variety, ovum (we'll use the less-precise egg cell instead) is the female.  The differences in these cells will ultimately tell you if an organism or a reproductive organ is male or female, and it is the differences between the cells that really leads to the larger differences between males and females in any species that has genders...

SPERM	EGG CELLS
...are produced in much much higher quantities.	...are produced in comparatively lower quantities.
...are much smaller than egg cells.	...are much larger than sperm, since they contain food for the embryo.
...are equipped to get from where they're made to where the egg cells are.	...pretty much wait in one place for the sperm to reach them.
...from each starting cell that divides by meiosis, four functional sperm are made.	...from each starting cell that divides by meiosis, one functional egg cell is made, with 3 tiny polar bodies used to discard "extra" sets of chromosomes.

Some things to remember:  don't assume that things always work the way that they do in humans, or even in animals.  In most gendered organisms, lots of egg cells are made (it's just that way more sperm are made), not just one (which is a commonly-known "fact" that isn't even really true for people).  And although many sperm types can swim or crawl, many (like pollen) can't, even though they have evolved ways to get where they need to go.  And forget "XX" and "XY" chromosome mixes - gender is coded by chromosomes only in some animals, and most of the time there is no chromosomal difference between males and females.  In addition, many types of adult organisms are both male and female at the same time, a condition that used to be called hermaphroditism but is now called being monoecious.

SEX WITHOUT GENDER

You might wonder how organisms can reproduce sexually and not have male and female forms.  This approach is common in funguses, which generally exist as a growing mass of fibers or tendrils.  When two growing fungi encounter each other, they will extend fibers toward each other, and where they touch, the cells will produce haploid nuclei that they swap, like sperm fertilizing egg cells except with no real differences between the fibers.  Those connecting cells then use the new haploid nuclei, producing new cells that are a mixture of chromosomes different from their parents, and grow into new and genetically distinct fungi.

Informative Links

A simple introduction to mosses.

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

Lichens
Bryophyte types
Vascular Plants
Nonvascular Plants
Moving water in nonvascular plants
Diffusion
Adhesion
Alternation of Generations
Sporophyte
Gametophyte
Moss Life Cycle
Haploid
Rhizoids
Antheridium
Archegonium
Diploid
Meiosis
Gametes
Male / Female Differences
Monoecious

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