Chapter 27

1. The broadest use of the term “plant” refers to eukaryotes that descended from the ancestor with the first primary endosymbiosis of chloroplasts: glaucophytes, red algae, the various lineages of green algae, and land plants. Another use of the term refers to green plants (those that contain chlorophyll b, or the green algae and land plants). The most common use of “plant” refers only to land plants.

2. “Algae” is a convenience term that is used to refer to many distantly related, photosynthetic, aquatic organisms. This includes many groups of eukaryotes outside the Plantae.

   A-29

3. Glaucophytes are unicellular and also differ from the other groups because their chloroplasts contain a small amount of peptidoglycan between the inner and outer membranes. Most red and green algae are multicellular. Red algae contain an accessory photosynthetic pigment (phycoerythrin), which often gives them a red color, especially in deeper water. The green algae lack phycoerythrin but have a second kind of chlorophyll (chlorophyll b) in addition to chlorophyll a, which is also found in glaucophytes and red algae.

4. Both land plants and green algae contain chlorophyll b in addition to the chlorophyll a also found in glaucophytes and red algae. Land plants are more closely related to some green algae lineages than to others. The green algae most closely related to the land plants are the stoneworts; these two groups share protected embryos, a cuticle, a multicellular sporophyte, the presence of gametangia, and thick-walled spores.

1. Alternation of generations is a life cycle in which a multicellular diploid sporophyte generation produces spores by meiosis, alternating with a multicellular haploid gametophyte generation that produces gametes by mitosis. Humans (and other animals) do not have alternating sexual and asexual multicellular stages. They undergo meiosis to produce sex cells and use mitosis for growth and maintenance, not for reproduction.

2. Terrestrial adaptations found in all major lineages of land plants (including liverworts, mosses, and hornworts) include the cuticle, which provides a coating of waxy lipids that retards water loss; gametangia (multicellular organs that enclose plant gametes), which prevent gametes from drying out; protected embryos; pigments that provide protection against ultraviolet radiation; thick-walled spores; and a mutually beneficial association with fungi that promotes nutrient uptake from the soil. Stomata, which are small closable openings in leaves and stems that are used to regulate gas exchange, are a terrestrial adaptation that is found in all land plants except liverworts.

1. Xylem conducts water and minerals from the soil to aerial parts of the plant. Xylem also provides structural support. Phloem conducts the products of photosynthesis from sites of production to sites of storage within the plant.

2. The earliest rootlike structures were rhizomes—horizontal portions of stem that bear water-absorbing filaments called rhizoids. Rhizomes are retained in some modern plants, such as liverworts and mosses, and are also found in some vascular plants, such as ferns and lycophytes. Ferns and lycophytes, along with other vascular plants, also have true roots, a specialized organ for extracting water and minerals from the soil. Leaves may have originated twice among the vascular plants. The simple microphylls of lycophytes may have arisen from sterile sporangia. In contrast, the larger and more complex megaphylls, or true leaves, may have arisen from the flattening of a portion of a branching stem system that exhibited overtopping growth.

3. Heterospory refers to the presence of two distinct types of spores, one that develops into a female gametophyte and the other into a male gametophyte. One advantage of heterospory is that it promotes outcrossing among individuals. The fact that heterospory has evolved multiple times from homospory provides evidence that the heterosporous condition is advantageous.

1. 

2. Changes for each branch are indicated on the tree diagram provided with the answer to Question 1. Note that there are alternative (equally parsimonious) reconstructions for character numbers 6, 14, and 16.

3. Characters 7 and 19 exhibit homoplasy.

4. These groups are labeled in the figure provided as the answer to Question 1.

1. The 95% confidence intervals for the means can be calculated by multiplying the standard errors of the means by 1.96, and then adding and subtracting this product from the respective mean.

   The 95% confidence limits for each measure of chlorophyll a concentration (mg/L) in the various growth media are as follows:

   Growth medium only	Growth medium + propionate	Growth medium + acetate	Growth medium + citrate	Growth medium + sucrose	Growth medium + glucose
   10.98– 15.84	1.66–1.78	7.79–11.01	14.10– 16.42	15.94– 19.32	29.84– 44.58

   Chlorophyll a concentrations in growth medium + propionate are clearly lower than in growth medium alone, as the 95% confidence intervals do not overlap. There is almost no overlap between the growth medium only and growth medium + acetate conditions, with the added acetate showing lower chlorophyll a concentrations. There is substantial overlap in the confidence intervals for growth medium alone and growth medium + citrate, suggesting no significant difference in those two conditions. The other two conditions, with sucrose or glucose added to the growth medium, result in significantly higher chlorophyll a concentrations than does growth medium alone.

2. A-30

   The measures of chlorophyll a concentration and biomass are strongly correlated, whereas cell counts are not as strongly correlated with these other measures. The reason is that favorable growth conditions produce large cells (with greater biomass) that contain proportionally more chlorophyll a, whereas poor growth conditions produce small cells that contain proportionally less chlorophyll a. Although poor growth conditions sometimes result in fewer cells than favorable growth conditions, the growth conditions have a greater effect on cell size than on cell numbers.

   

1. The last common ancestor of seed plants and ferns lived in the mid-Devonian about 390 million years ago.

2. Spikemosses did not exist until the late Devonian. Spikemosses were not yet present when the last common ancestor of seed plants and ferns lived and thus fossils of spikemosses would not be expected in the same stratum with fossils of the common ancestors of ferns and seed plants. Because the common ancestor of ferns and seed plants lived before spikemosses did, we would expect fossils of that ancestor to be in a lower stratum.

3. Yes, rhyniophytes existed from the late Silurian through much of the Devonian, and survived for tens of millions of years after the last common ancestor of seed plants and ferns lived. So it would not be surprising if fossils of these two lineages were in the same stratum.

4. The last common ancestor of embryophytes lived in the Ordovician about 470 million years ago. This lineage was the first group of plants to live out of water and colonize land.