How Plants Colonized Land Chapter 29

Overview: Origins of Mulitcellularity Quick look at over all diversity in Plant Kingdom Characteristics of Plants Special features, structures Origin from green algae Link to Charophyceans Evidence to support Dates Alternation of generations Origin of embryophytes Bryophyte (moss) lifecycle – in Lab Pteridophyte (fern) lifecycle – in Lab

Multicellularity Arose independently several times in Eukarya Caused another new wave in evolution Origins in simpler colonial forms – Volvox Cellular specialization and Division of labor Escape cell size limitations Membrane area to cytoplasm volume ratio

Multicellularity solves ratio limits Fig 6.7

The Plant Kingdom Origins over 475 MYA 10 Divisions 4 Basic lifecycles Green algae that evolved onto land Evolved becoming more terrestrial, independent from water Then coevolved with pollinators, dispersal

Seedless vascular plants An overview of land plant evolution Land plants Vascular plants Bryophytes (nonvascular plants) Seedless vascular plants Seed plants Mosses Liverworts Hornworts Charophyceans Gymnosperms Angiosperms Pterophyte (ferns, horsetails, whisk fern) Origin of seed plants (about 360 mya) Lycophytes (club mosses, spike mosses, quillworts) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga Figure 29.7

Major events: Living on land, spores Vascular tissues Pollen, Seeds Flowers & Fruits

The Plant Kingdom? Who is in / out? Plantae Streptophyta Viridiplantae Red algae Chlorophytes Charophyceans Embryophytes Ancestral alga

We’ll see all but the hornworts in lab Table 29.1 We’ll see all but the hornworts in lab

Evolutionary Sequence Green Algae (Charophyceans) gave rise to: Mosses (475 MYA) and liverworts, which gave rise to: Ferns (420 MYA) and related plant groups which gave rise to: Gymnosperms (360 MYA) Conifers and related plant groups which gave rise to: Angiosperms (140 MYA) (flowering plants) which have form two groups: Monocots Eudicots

Charophyceans- plant’s green algae ancestor Chara Coleochaete Modern examples of charophyceans

Original Traits Derived Traits Found in ancestral species and new species Not found in ancestral species, new to daughter species Older terms: primitive and advanced traits

What’s new in Plants: Some Derived Characteristics of Plants Growth by divisions in Apical Meristems Multicellular dependent embryos Alternation of generations Spores Multicellular gametangia Cuticle Transport tissues Secondary compounds

Growth by cell divisions in Apical Meristems Localized regions of cell division In shoot tips, roots

Multicellular dependent diploid embryos Land plants are called embryophytes Haploid charophyceans

Spores Walls of resistant sporopollenin Dry out and travel in the wind Zygotes of Charophyceans protected by sporopollenin – precursor to spore walls? Disperse then grow into gametophyte plants

Multicellular gametangia Archegonia Antheridia Make egg at base of produce many sperm Vase like column cells that swim to egg All haploid tissues, gametes form by mitosis

Cuticle Waxy covering layer prevents water loss, and microbial attack Stomata allow for gas exchange Thicker layers in plants adapted to arid conditions

Transport tissues Xylem carries water up from the roots to the leaves Phloem carries a sugary solution through out the plant

Secondary Compounds Metabolic side branches off common pathways Provide benefit to plant ( defense, etc.) Can be used by People as Flavorings, drugs insecticides, etc.

APICAL MERISTEMS ALTERNATION OF GENERATIONS of shoot Developing leaves 100 µm Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the tips of a shoot and root. Apical meristem of root Root Shoot Haploid multicellular organism (gametophyte) Mitosis Gametes Zygote Diploid multicellular organism (sporophyte) Alternation of generations: a generalized scheme MEIOSIS FERTILIZATION 2n n Spores ALTERNATION OF GENERATIONS

WALLED SPORES PRODUCED IN SPORANGIA Sporangium Sporophyte and sporangium of Sphagnum (a moss) Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte MULTICELLULAR GAMETANGIA Female gametophyte Archegonium with egg Antheridium with sperm Archegonia and antheridia of Marchantia (a liverwort) Male gametophyte MULTICELLULAR, DEPENDENT EMBRYOS Embryo Maternal tissue 2 µm Embryo and placental transfer cell of Marchantia 10 µm Wall ingrowths Placental transfer cell

Origin of plants How did the sporophyte generation come about? Preadaptations may have lead the charyophyceans onto land

Hypothesis for origin of alternation of generations Delayed meiosis maximizes output of sexual reproduction. More spores can be produced per fertilization event Adapting to drying conditions with fewer surviving spores Lineages separate before sporophytes evolved much

Preadaptation Evolutionary adaptation that was selected for under one set of conditions This trait then gives an advantage for a new situation with different conditions.

Preadaptation Evolutionary adaptation that was selected for under one set of conditions This trait then gives an advantage for a new situation with different conditions. Feathers arose first for insulation, then helped with flight

Adaptation to shallow waters preadapted Charophyceans to life on land Adapted to periodic drying during low tides, droughts Leads to cuticle? Adapted to higher light intensities Leads to common chloroplast structure? Zygote protected from drought within archegonia with a layer of sporopollenin Leads to spore wall?

Alternation of Generations Separate multicellular haploid and diploid phases (2n) Sporophyte make spores by meiosis (n) Gametophyte makes gametes by mitosis Sperm and egg (moss & fern) Pollen and Ovule (gymnosperm & angiosperm) The sporophyte and gametophyte are very different in morphology Vascular tissues only appear in sporophyte phase Sporophyte becomes more dominant in new plant groups Charophyceans lack sporophyte phase

Charophycean life cycle

Characteristics that Plants share with the green algae group Charophyceans Autotrophic Multicellular Eukaryote Have cell walls made of cellulose Made by rosette cellulose-synthesizing complexes 20-26% of wall material, closest match in algae Chloroplast similarities have chlorophyll a & b, use β-carotene as accessory Thylakoids stacked in grana Chloroplast DNA comparisons Peroxisome enzymes Cell plate formation by phragmoplast Nuclear membrane breaks down during mitosis Sperm ultrastructure - biflagellate Gene sequences – rRNA, Cytoskeleton proteins

Switch to sporophyte dominance

Figure 29.13 Hypotheses for the evolution of leaves Vascular tissue Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue. (a) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems. (b)

What’s new in Mosses? Spores / sporangia Sporophyte phase Upright growth on land Cuticle Multicellular gametangia

The Bryophytes Bryophytes are represented by three divisions: Division Hepatophyta - liverworts Division Anthocerophyta - hornworts Division Bryophyta – mosses Liverworts and hornworts are believed to be more similar to what early plants were like.

Bryophyte lifecycle: moss Haploid dominant No vascular tissues Filamentous protonema stage Swimming sperm Disperse by spores Dependent sporophyte Dioecious gametophytes No true leaves Rhizoids, not roots

What’s new in Ferns? Vascular tissue True roots and stems and leaves Stomata Sporophyte (2n) dominate

Pteridophytes evolved over 400 MYA Seedless, Vascular plants (having Xylem & Phloem). Today represented by two divisions: Pterophyta: Ferns, Horsetails (Equisetum) Lycophyta: Club moss Cooksonia, an extinct plant over 400 million years old, is the earliest known vascular plant. The branched sporophytes were up to 50cm tall with small lignified cells, much like the xylem cells of modern pteridophytes.

Vascular tissue Allows plants to grow taller More support by lignified xylem tracheids Can pull water up from soil Can tolerate soil that is drier on the surface Form parts of true leaves and roots. Only found in diploid tissue Lead to sporophyte dominance?

Fern Lifecycle Diploid dominate Gametophyte still independent, short lived, monoecious in fern (Pteridophyta) dioecious in club moss (Lycophyta) Sporophyte in Lycophyta is Monecious in Pteridiophyta is Dioecious Spores disperse plant Sporophyte perennial, monoecious

Figure 29.23 The life cycle of a fern

LYCOPHYTES (PHYLUM LYCOPHYTA) PTEROPHYTES (PHYLUM PTEROPHYTA) WHISK FERNS AND RELATIVES HORSETAILS FERNS Isoetes gunnii, a quillwort Selaginella apoda, a spike moss Diphasiastrum tristachyum, a club moss Strobili (clusters of sporophylls) Psilotum nudum, a whisk fern Equisetum arvense, field horsetail Vegetative stem Strobilus on fertile stem Athyrium filix-femina, lady fern

Carboniferous forest based on fossil evidence