CHAPTER 29
Concept 29.1: Land plants evolved from green algae
• Green algae called charophytes are the closest relatives of land plants
Morphological and Molecular Evidence
• Many characteristics of land plants also appear in a variety of protist clades, mainly algae
• However, land plants share four key traits with only charophytes
–Rings of cellulose-synthesizing complexes
–Peroxisome enzymes
– Structure of flagellated sperm
–Formation of a phragmoplast
• Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives
of land plants
• NB. land plants are not descended from modern charophytes, but share a common ancestor with
modern charophytes
Adaptations Enabling the Move to Land
• In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from
drying out
• Sporopollenin is also found in plant spore walls
• The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO2,
nutrientrich soil, and few herbivores or pathogens
• Land presented challenges: a scarcity of water and lack of structural support
• The accumulation of traits that facilitated survival on land may have opened the way to its colonization
by plants
Derived Traits of Plants
• Four key traits appear in nearly all land plants but are absent in the charophytes
– Alternation of generations and multicellular, dependent embryos
–Walled spores produced in sporangia
–Multicellular gametangia
–Apical meristems
Alternation of Generations and Multicellular, Dependent Embryos
• Plants alternate between two multicellular stages, a reproductive cycle called alternation of
generations
• The gametophyte is haploid and produces haploid gametes by mitosis
• Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis
• The diploid embryo is retained within the tissue of the female gametophyte
• Nutrients are transferred from parent to embryo through placental transfer cells
• Land plants are called embryophytes because of the dependency of the embryo on the parent
Walled Spores Produced in Sporangia
• The sporophyte produces spores in organs called sporangia
• Diploid cells called sporocytes undergo meiosis to generate haploid spores
• Spore walls contain sporopollenin, which makes them resistant to harsh environments
Multicellular Gametangia
• Gametes are produced within organs called gametangia
• Female gametangia, called archegonia, produce eggs and are the site of fertilization
• Male gametangia, called antheridia, produce and release sperm
Apical Meristems
• Plants sustain continual growth in their apical meristems
• Cells from the apical meristems differentiate into various tissues
