Plant Origins and Adaptations

Plant Origins

• Plant ancestors were most likely charophytes, a type of green algae
• Charophytes share key traits with plants: chlorophyll a and b, beta carotene, similar thylakoid arrangements, identical cell walls, starch as a storage carbohydrate, and similar sperm and peroxisomes

Life on Land

• Plants needed to develop certain adaptations to thrive, including roots, vascular tissue, lignin and sporopollenin, cuticle development, and stomata development
• CO2 and light are abundant terrestrial resources

Characteristics of Plants

• Plants are Eukaryotic and Multicellular
• Plants are photosynthetic using chlorophyll a and b
• Plant cell walls are made of cellulose
• Plants are sessile and form an embryo enclosed in maternal tissue
• Plant life cycle follows the Alternation of Generation, this includes the Sporophyte and Gametophyte Generations
• Sporophyte generation is diploid, produces haploid spores by meiosis
• Spores grow into the gametophyte and are produced in sporangia
• Gametophyte generation is haploid producing haploid gametes by mitosis
• Gametes eventually fuse to form a diploid zygote that grows into a sporophyte
• Gametes form in gametangia which includes Antheridia that produces sperm (male) and Archegonia which produces eggs (female)

Plant Evolution

• Sporophyte generation has become more dominant in plant evolution and Gametophyte has become microscopic
• Vascular systems have developed
• Seeds developed to protect the embryo
• Flowers and fruits are used as reproductive structures

Plant Adaptations

• Plants have developed defense mechanisms due to being unable to escape plant eating animals
• Some plants have spines, spikes, and thorns that deter predators
• Some trees have thick layers of bark that are shed in order to get rid of attacking insects
• Conifers exude pitch, a sticky substance that can engulf and smother attacking insects
• Locoweed, which occurs all over western North America contains substances that when eaten, can cause cattle and horses to become lethargic and stop feeding
• Some plants synthesize chemicals that induce physiological and behavioral changes in animals that eat them
• Plants have developed ways of getting the male gamete to the female gamete, including alternating haploid and diploid life stages, and using other organisms to transport the gametes
• Plants have adaptations such as thorns to defend themselves because they are unable to run from predators
• Plants bend in place and grow toward light so they can obtain food since they cannot move to reach sunlight

Plant Divisions

• Plants are divided by their adaptations to land living and these Divisions include Bryophytes, Vascular Seedless Plants, Gymnosperms and Angiosperms

Bryophytes

• Bryophytes lack true roots, stems and leaves, instead they have rhizoids
• Bryophytes are considered non-vascular and rely on diffusion
• Asexual reproduction predominates in Bryophytes via Fragmentation, however, sexual reproduction uses archegonia and antheridia
• Gametophyte is the dominant form in Bryophytes (haploid)
• Sporophyte grows out of the gametophyte after sperm & egg unite in the archegonia
• Sporophytes (2n) produce spores (n) via meiosis within a sporangium called a capsule
• There are three major groups of Bryophytes these are Mosses, Liverworts & Hornworts

Vascular Seedless Plants

• Vascular Seedless Plants are Horsetails, Club Mosses & Whisk Ferns
• Vascular Seedless Plants have vascular tissue, cuticle
• Vascular Seedless Plants rely on watery environments for reproduction

Ferns

• Sporophyte Generation (fronds) produces haploid spores via meiosis in a sporangium called a sorus (sori)
• Some Ferns are heterosporus with Microspores give rise to male gametophytes and Megaspores give rise to female gametophytes
• Gametophyte Generation (Prothallus) has to vascular tissue in the prothallus
• Archegonia & antheridia can be found on the same gametophyte

Gymnosperms

• Gymnosperms vascular tissue, pollen and seeds (by fertilization)
• Gymnosperms have needle like leaves and Reproductive cones
• Gymnosperms undergo Gametophyte Generation where Male gametophyte is pollen and Female gametophyte is reduced
• Gymnosperms under go Sporophyte Generation where the Embryo is protected by a seed coat and stays dormant until germination

Coniferophyta

• Coniferophyta is a type of Gymnosperm that as vascular tissue, pollen and seeds
• Gametophyte Generation includes Male gametophyte which is winged pollen and contains the tube cell. The Female gametophyte is found in female cones and the embryo and seed develop together
• The Sporophyte Generation (diploid) produce cones, male and female, also spores give rise to the gametophyte within the sporophyte

Angiosperms

• Angiosperms have “covered seeds” and are vascular plants with a more efficient xylem and have Broader leaves
• two major groups: Monocots & Dicots
• Angiosperms produce flowers that Contain the anther and carpel and are More efficient for pollen dispersal as the Ovary develops into a fruit with seeds inside