Chapter 30 Seed Plants

Reproductive adaptations of plantsGymnosperms

Why are seed plants more successful?Plant diversity 2

First seed plants 350 million years ago Sporpollenin Lan plants 475 million years ago Vascular plants 425 million years ago Extant seed plants 305 million years agoHeterospory and reduced gametophytes Seed plants are heterosporous Megaspores -> female gametophytes Microspores -> male gametophytes Spores remain within sporophyte tissue Growing into tiny M or F gametophytes Why is this an adaptation? Gametophytes dont have the moisture they need to grow without drying out Mosses and other non vascular -> gametophyte dominant Ferns and other seedless vascular plants -> sporophyte dominantOvule Derived from a megasporangium on a sporophyll The megaspore -> female gametophyte Egg produced and fertilized here The ovule includes Integument (2n) Megasporangium (2n) Megaspore (n) Female gametopohyte (n)Pollen The microsporangium retains microspores Microspores -> tiny male gametophytes in a protective (sporopollenin) coating = pollen grains Pollen grains travel through the air to reach ovule bearing plants (NEVER SEEN BEFORE) New step in sex = pollination *** Pollen grain then grows a pollen tube* toward the egg within the ovule The sperm is released within the pollen tube No flagella in conifers and angiosperm ***chemotaxis so the tube knows which direction to grow Fertilization occurs within the ovule Note: no rain or dew or moisture neededSeed Develops from fertilized ovule Includes seed coat, food supply (n), embryo (2n) Seed enhances survival on land Efficient adaptive dispersal Underground germination possible Stored food allows embryo growth without photosynthesis May stay dormant until conditions are OKGymnospermsThe naked seed plants

Which plant adaptation is absent in both ferns and mosses?PollenWhat is the function of pollen?Deliver sperm to the egg

Ovules borne exposed on sporophylls Usually arranged in cones No flower; no fruit Phylum gingkophyta One extant sp.: biloba Herbal medicine Flagellated sperm Phylum cycadophyta Form like a palm tree about 100 species Ex: zamia Ex: sago palm Central male or female cones Flagellated sperm Phylum gentophyta 3 distinct genera about 70 species Sperm not motile Gnetum in tropics Does not have sperm; similar to conifers Ex: ephedra in US deserts Phylum Coniferophyta Most diverse gymnosperms today Cone bearers Male pollen cone (sperm not motile) Female ovulate cone Dominate high altitudes and latitudes in N. Hemisphere Most are evergreen with needle like leaves EX: pine, fir, spruce, cedar, juniper, cypress Record breakers: tallest organisms, most massive organism Life Cycle of a Pine Tree 3 key productive adaptations Increasing dominance of the sporophyte The advent of the seed as a resistant, dispersible stage The appearance of pollen as an airborne agent Sporophyte -> pine tree Sporangia -> in the scales of the cone Two types of spores produced by separate cones Small pollen cones Microsporocytes undergo meiosis producing haploid microspores -> microspore develops into pollen grain, containing the male gametophyte Large ovulate cones Megasporocytes undergo meiosis and produce haploid megaspores inside the ovule. Surviving megaspores develop into multicellular female gametophytes (retained within the sporangium) It takes nearly three years, for the male and female gametophytes to be produced and brought together and for mature seeds to form from fertilized ovulesAngiospermsClassified into a single Phylum: Anthophyta Derived Characteristics Flower: structure specialized for sexual reproduction Flowers contain ovaries, which surround and protect the seeds In general, an advantage because they promote the transport of pollen by being attractive to other animals and insects A specialized shoot Fruit After pollination, the ovary develops into a fruit, which protects the seed and aids in its dispersal a wall of tissue surrounding the seed Fruit increase the ranges they (angiosperms) in habit, spreading the seeds over hundreds of square kilometers About the flower: They can vary in: Shape and color Smell and nectar Organ arrangement and time of blooming (opening) Flowers are determinate shoots; they cease growing after the flower and fruit are formed 4 rings of modified leaves (sporophylls) called floral organs Floral organs include the sepals, petals, stamens, and carpels Sepals enclose the flower before it looms, base of the petals Petals: Interior to the sepals Brightly colored Aid in attracting pollinators *flowers that lack brightly colored petals -> wind pollinated Sepals and petals are sterile (do not produce sperm or eggs) Within the flower Two whorls of fertile floral organs that produce spores Stamens and Carpels Stamens Produce microspores -> pollen grains containing male gametophytes Consists of: A stalk, called a filament And a terminal sac, the anther (where pollen is produced by microsporangia) Carpels Make megaspores and their products (female gametophytes) Some flowers have a single carpels, others have multiple ones A sticky stigma, at the tip of the carpel receives pollen A style leads from the stigma to the ovary at the base of the carpel *if fertilized, an ovule develops into a seed pistil sometimes used to refer to a single carpel or two or more fused carpels Complete flowers have all 4 basic floral organs Some species have incomplete flowers Lacking sepals, petals, stamens, or carpels Ex: most grass flowers lack petals Fruit Consists of a mature ovary but can also include other flower parts As seeds develop from ovules after fertilization, the ovary wall thickens Fruits protect dormant seeds and aid in dispersal Fruits can be fleshy or dry Dry fruits Include beans, nuts, and grains Some split open at maturity to release seeds, whereas others remain closed Wind-dispersed typically Cereal grains of maize, rice, or wheat are actually a fruit with a dry outer covering (the former wall of the ovary) Fleshy fruits Grapes, plums, tomatos The wall (pericarp) of the ovary becomes soft during ripening Fruit adaptations Seeds of some flowering plants, are contained within fruits that function as parachutes or propellers Some, like coconuts, are adapted to disperse by water Some seeds have fruits modified as burrs that cling to animal fur Edible fruits, when eaten, are digested and the animal deposits the seeds with its feces (natural fertilizer

Life cycleOf Angiosperms In angiosperms, the sporophyte is the dominant generation: larger, more conspicuous, and longer lived than the gametophyte The diploid plant, the sporophyte, produces haploid spores by meiosis These spores divide by mitosis, giving rise to multicellular gametophytes (the male and female haploid plants that produce gametes (sperm and egg)) Fertilization, the fusion of gametes, results in diploid zygotes, which divide by mitosis and form new sporophytes The flower of the sporophyte produces microspores that form male gametophytes, and megaspores that form female gametophytes Male gametophytes Within the pollen grains, which develop within microsporangia in the anthers Each male gametophyte has 2 haploid cells; a generative cell that divides into 2 sperm, and tube cell that produces a pollen tube Together, these 2 cells and the spore wall constitute a pollen grain Female gametophytes Each ovule, which develops in the ovary, contains a female gametophyte, known as the embryo sac The embryo sac consists of only a few cells, one of which is the egg After pollen is released from the anther, it travels till it becomes stuck to the stigma at the tip of the carpel ***Although some flowers self-pollinate, MOST have mechanisms that ensure CROSS-POLLINATION Cross-pollination enhances genetic variability In some flowers, the stamen (male) and carpel (female) develop and mature at different rates; or they may be physically arranged so self-pollination is unlikely Ex: fused anthers They can also be unisex The pollen grain (male) absorbs water and germinates after it adheres to the stigma of a carpel The tube cell (male) produces a pollen tube that grow down within the style of the carpel After reaching the ovary, the pollen tube penetrates the micropyle (pore in the integuments of the ovule) and discharges two sperm cells into the embryo sac One sperm fertilizes the egg -> diploid zygote The other fuses with the two nuclei in the large central cell of the female gametophyte, producing a triploid cell (endosperm) DOUBLE FERTILIZATION, one produces a zygote, and another produces and triploid endosperm**** unique to angiosperms After DF, the ovule matures into a seed Zygote develops -> sporophyte embryo with a rudimentary root and one or two seed leaves called cotyledons The triploid central cell of the female gametophyte develops into endosperm***, tissue rich with starch and other food reserves that nourish the developing embryo Double Fertilization what is its function? Synchronizes the development of food storage in the seed with the development of the embryo Prevents flowers from squandering nutrients on infertile ovules Pollination The transfer of pollen from the anther to the stigma Accomplished by wind, water, or animals