Activity 29/30.2 What can a study of extant species tell
us about the evolution of form and function in the plant
kingdom?
Fill in the chart on the next pages to compare the major features of key groups of land
plants with one another and with the charophyceans.
Use the information in the chart to answer the questions.
1. Some of the major plant groups are listed in the following chart from most primitive to
most advanced. For each group, indicate what major characteristics make it more advanced
than the preceding group. For example, how are ferns more advanced than mosses?
2. How do the bryophytes differ from the seedless vascular plants? How are they similar?
Refer to the table above. Differences between bryophytes and the seedless vascular plants
are:
a. the dominant phase of the life cycle
In bryophytes the gametophyte stage is dominant; in the seedless vascular plant the
sporophyte is dominant.
b. the absence/presence of vascular tissue.
Bryophytes contain no vascular tissue. As a result, they do not have true stems, roots
or leaves.
c. the absence of guard cells in bryophytes.
Both bryophytes and seedless vascular plants have air spaces in their structure for gas
exchange. However, the bryophytes do not have guard cells associated with their air
spaces. Guard cells allow plants to control the size of the stomata.
In most other major features (listed in the table above), these two groups are similar.
3. The life cycle of all land plants includes an alternation of generations between a
multicellular gametophyte phase and a multicellular sporophyte phase.
Diagram the life cycle of a seed plant.
Refer to Figures 30.6 and 30.10 in Biology, 7th edition. See also Figure 13.6 on page 242.
a. What cellular division process always precedes formation of the gametophyte
generation?
Meiosis
b. What cellular division process always precedes formation of the sporophyte
generation?
Fertilization forms the zygote (the first cell in the sporophyte generation), and mitosis
of the zygote forms the multicellular sporophyte.
c. If the sexual generation gives rise to the gametes, what part of an angiosperm is
sexual?
The gametophyte generation, which is haploid, gives rise to the gametes and is
therefore the sexual generation.
d. If the sexual generation gives rise to the gametes, what part of a bryophyte moss is
sexual?
The gametophyte or green, leaflike, moss structures give rise to the gametes in
archegonia and antheridia.
4. Until the evolution of the seed plants, land plants were dependent on the availability of
water for reproduction. Explain why this was true. Explain how seed plants overcame the
need for water in reproduction.
Seed plants produce pollen rather than flagellated sperm. Pollen do not require water to
move from one plant to another. Pollen can be carried by the wind or by animals to other
plants.
5. Pollen, seeds, flowers, and fruits are considered among the most advanced
characteristics in the plant kingdom. What evolutionary advantage(s) does each of these
offer (relative to what existed before)?
Pollen: As noted in question 4, pollen no longer requires free water for transport.
Seeds: The seed contains a partially developed 2n embryo (the product of fertilization of
the egg by a sperm from the pollen tube), a food store for its early development, and a
protective coating (derived from the integuments surrounding the ovule) that allows it to
survive periods of desiccation. You can think of the seed as an embryo, with a packed
lunch, and a raincoat that is sent out into the world to make it on its own.
A seed has two main advantages over a spore: The embryo is in a more advanced state of
development, and the larger food store both allows for longer periods of dormancy and
supports early growth phases of the seedling following germination. Upon germination,
the food store in the seed allows it to continue developing roots, stem, and leaves until the
plant breaks through the soil surface and is capable of photosynthesis.
Flowers: The flower serves protection and nourishment functions for both the
gametophyte (the embryo sac as it is developing) and the next-generation sporophyte (the
developing seed after fertilization occurs). In addition, the flower often serves to attract
specific pollinators that increase the possibility of successful pollination.
Fruits: The fruit serves no function for the developing embryo or the seed once formed.
In fact, the fruit tends to contain hormones that prevent germination of the seeds (that is,
maintain seed dormancy). Only after the fruit has rotted away (or is eaten away) can the
seeds germinate. The fruit is generally thought to be a mechanism used by angiosperms
to coopt or bribe animals to disperse their seeds (contained in the fruit). In some cases,
this system has evolved to the point that certain species’ seeds will not germinate unless
they have passed through the digestive tract of a particular animal species.