The Plant Kingdom Evolution from Water to Land

Primitive Plants Were “aquatic” – lived in water If salt water, we use the term “marine” It is believed that land plants evolved from simple, algae-like, aquatic plants. We think plants and algae are closely related because they both: Have the same kind of chlorophyll Store their food as starch Have cellulose in their cell walls

WHY did ancestral plants live in the water? Prevents them from drying out They don’t need roots to soak up water Reproductive gametes can easily swim from male to female sex organs No need for strong structures to hold them up… buoyancy of water supports them

Characteristics of Plants  multicellular  photosynthetic  adapted for living primarily on land  non-motile (motile means capable of motion)  Cell walls composed of cellulose  Have chloroplasts with chloropyll  Food stored as starch

Alternation of Generations All plants show ALTERNATION OF GENERATIONS in their life cycles 2 stages: Sporophyte stage (diploid cells) Gametophyte stage (results from meiosis and produces haploid gametes) In trees, the part we see is the sporophyte. The gametophyte is very small and internal.

How are plants classified? There are – classified plants on earth. They are divided into 2 groups, according to adaptations for life on land: Non vascular plants (mosses, liverworts and hornworts) smaller, and dependent on moist conditions to live Vascular plants (trees, shrubs, grasses) live on the land

Non Vascular Plants Mosses need water for reproduction – male gamete swims to female gamete through water Do not have true roots, stems or leaves, but have structures similar to vascular plants Stay small in size because diffusion is how they get their water and nutrients – every cell must be near water source. See this site for a picture.this site

Moss

Moss Life Cycle

Life Cycle of Moss gametophyte stage – consists of cells that are haploid (n). some gametophyte cells produce eggs; others produce sperm. In a moist environment water carries the sperm to the eggs. Fertilization  diploid (2n) zygote  grows into a structure called a sporophyte and remains on the female gametophyte sporophyte generation is diploid and produces haploid spores through meiosis haploid spores land on moist ground -- grow into a gametophyte

Vascular Plants These include all land plants Gymnosperms (cone-bearing plants) Angiosperms (flowering plants) In the vascular plants, the sporophyte is the dominant generation The gametophyte is found inside the seeds of the plant.

Angiosperms & Gymnosperms have highly specialized reproductive structures, producing the embryo (the gam- etophyte) protected inside seeds….

These seeds are fertilized inside the female parts of a flower…

Advanced water and nutrient carrying tissues, XYLEM & PHLOEM: Cross sectionLongitudinal section

Specialized Leaves

Specialized Leaves: Conifer needle cross section

Openings called stomata reduce water loss while allowing for gas exchange….

Summary of Plant Adaptations for Life on Land: Roots allow for water absorption from deep in the soil when rainfall is scarce Protective mechanisms prevent water loss – cuticle (waxy coating) on leaves, shape of leaves (ie. Needles) Strong stems and structures to hold the plant up against gravity--necessary without the buoyancy of water to hold the plant up.

Summary continued… Stomata, pores in the leaves, allow for gases to enter or exit the interior of the leaf (plants use CO 2 and give off O 2 ) Water is “sucked up” in xylem tubes, and nutrients are transported in phloem tubes to parts of the plant that don’t photosynthesize, so the need for small plant bodies for the sake of diffusion is reduced

Summary continued… Thickened cell walls for rigid support Extreme fluctuations in temperature, humidity, sunlight and wind require adaptations as well. Cacti, evergreen trees, prairie grasses and shrubs all have adapted by means of water storage tissues, modified leaves to reduce water loss, etc.

The End