2 22.1 The Fungi: Life as a Web of Slender Threads
3 The FungiFungi do not make their own food, as plants do, but instead are heterotrophs—they consume existing organic material in order to live.
4 The FungiFungi consist largely of webs of slender tubes, called hyphae, that grow toward food sources.Collectively, hyphae make up a branching web, called a mycelium, that forms the bulk of most fungi.
5 The FungiFungi cannot immediately ingest the food the hyphae reach since only small molecules can pass through the cell walls of fungal cells.Fungi thus digest their food externally, through release of digestive enzymes, and then bring the resulting small molecules into the hyphae.
6 The FungiMany fungi have a reproductive structure called a fruiting body that produces and releases reproductive cells called spores.All fruiting bodies are organized collections of hyphae.
7 The Fungifruiting body (mushroom)sporehyphaemyceliumFigure 22.3
8 The FungiMost fungi are sessile or fixed in one spot, but their hyphae grow toward food sources.Such growth comes only at the tips of the hyphae, not throughout their length.
9 The Fungi Most fungi are multicellular. The primary exception to this, the single-celled yeasts, are thought to have evolved to the single-celled state from an earlier multicelled condition.
11 Roles of FungiFungi are the cause of many crop diseases, human infections, and forms of damage to buildings and homes.They are also sources of medicines and are used extensively in food processing.
12 Roles of FungiIn nature, fungi join bacteria as major decomposers of the living world.They break down the organic material in such objects as fallen trees and recycling the inorganic products that result back into the soil.
14 Structure and Reproduction in Fungi Individual cells that form hyphae often are separated from one another by dividers called septa.
15 Structure and Reproduction in Fungi Septa are porous enough to allow for a fairly free flow of cellular material between one hyphal cell and the next.Hyphae can bring resources very quickly to the point of growth at the hyphal tips.As a result, fungi can grow very quickly.
16 Structure and Reproduction in Fungi The fruiting body of the toadstool mushroom fungi, the mushroom cap, is made up on its underside of accordion-like gills.The gills produce spores and release them into the wind, which carries them to new locations.
17 Structure and Reproduction in Fungi Only a tiny fraction of fungal spores will successfully germinate, or sprout new mycelia.Because of this, fruiting bodies release huge quantities of spores as a means of ensuring new fungal growth.
18 Life Cycle of FungiMany fungi have a so-called dikaryotic phase of life in which the fusion of cells from different parental hyphae produces a single hypha with cells that each have two nuclei— one nucleus coming from one parental hypha, the other nucleus coming from the second parental hypha.Each of these nuclei is haploid, meaning each contains only a single set of chromosomes.
19 Life Cycle of FungiWhen the below-ground mycelium in a mushroom fungus sprouts into the above-ground mushroom cap, all the cells in the cap initially are dikaryotic.This includes those in the reproductive structure called the basidium.
20 Life Cycle of Fungi Figure 22.5 Fusion of nuclei (fertilization) gills Meiosis and spore formationdiploiddikaryoticbasidiumspore from another mushroomgillssporeshaploidLife cycle of a fungusdikaryotichyphaeGerminationMushroom formationdikaryoticFusionFigure 22.5
21 Life Cycle of FungiThe nuclei in certain cells of the basidia then undergo fusion, which is a step in carrying out sexual reproduction.These cells are now diploid—they have a single nucleus that contains a paired set of chromosomes.
22 Life Cycle of FungiThese same cells then undergo meiosis, and the result is four haploid spores attached to tip of the basidium.These spores are then released and blown by the wind to new locations, where they may germinate and result in new mycelia.
23 Life Cycle of FungiA spore is a reproductive cell that can develop into a new organism without fusing with another reproductive cell.Unlike human eggs and sperm, which fuse to make a fertilized egg, fungal spores do not need to fuse with any other cell to give rise to a new generation of fungi.
24 Life Cycle of FungiPLAYAnimation 22.1: Life Cycle of Fungi
26 Categories of FungiThere are four major categories or “phyla” of fungi:BasidiomycetesAscomycetesZygomycetesChytrids
27 Categories of FungiThe first three of these phyla are defined by reproductive structures.Any fungus that reproduces through use of the reproductive structure called a basidium is a basidiomycete, for example.The ascomycetes and zygomycetes are defined by their own reproductive structures.
28 BasidiomycetesThe basidiomycetes, also known as the club fungi, include “toadstool” mushrooms, “shelf ” fungi that often can be seen sprouting from trees, and the agricultural pathogens called smuts and rusts.
34 ChytridsThe chytrids are a primitive group of mostly aquatic fungi, all of which are mobile at one point in their life cycle.The chytrids probably give us an idea of what the earliest fungi were like, as the sessile fungi that are so common today seem to have evolved from chytrid-like ancestors.Early fungi were probably very similar to early animals.
36 YeastsA yeast is any single-celled fungus that tends to reproduce by the process known as budding.In this process, fungal daughter cells are produced as outgrowths of parental cells.
37 YeastsYeast species exist within three of the four fungal phyla—the ascomycetes, the basidiomycetes, and the zygomycetes.The majority of yeasts are ascomycetes, but many are basidiomycetes.
38 YeastsThe yeast Saccharomyces cerevisiae has been very important to human beings because of the critical roles it has played in the production of beer, wine, and bread and because of the important role it has played in scientific research.
40 22.5 Fungal Associations: Lichens and Mycorrhizae
41 LichensLichens are composite organisms made up usually of fungi and algae but sometimes of fungi and bacteria.
42 LichensWithin a lichen, the photosynthesizing algae are supplying the fungi with food, while the fungi are supplying the algae with water, minerals, carbon dioxide, and protection from the elements.
44 LichensLichens can establish themselves in barren environments, and some lichens have been alive for thousands of years.Lichens grow at an extremely slow pace, however, thanks to their practice of entering a state of dormancy during periods when water is scarce.
46 MycorrhizaeUp to 90 percent of seed plants live in a cooperative association with fungi that links plant roots with fungal hyphae.Associations of plant roots and fungal hyphae are called mycorrhizae.
47 MycorrhizaeIn this relationship, plants supply fungi with food produced in photosynthesis, while the fungi supply plants with minerals and water, gathered by the web of fungal hyphae.
50 Plants are the foundation for much of life on Earth because they are responsible for much of the living world’s production of food and oxygen.In addition, they stabilize soil, provide habitat for animals, and lock up carbon dioxide.
51 Characteristics of Plants All plants are multicelled, and almost all are fixed in one spot and carry out photosynthesis.All plant cells have a cell wall and contain organelles called chloroplasts, which are the sites of photosynthesis.
52 Characteristics of Plants cell wall (made of lignin and cellulose)cell membranenucleuscentral vacuole (contains mostly water)chloroplast (site of photosynthesis)Figure 22.15
53 Characteristics of Plants Plants reproduce through an alternation of generations: a life cycle in which successive plant generations produce either spores (the sporophyte generation) or gametes (the gametophyte generation).Within a given species, these two generations can differ greatly in size and structure.
54 The Alternation of Generations in Plants Human reproductionPlant alternation of generationsmeiosiseggmulticellular gametophytemulticellular diploid adultssporesgametessperm1n1nmeiosisfertilization2nfertilization2nmitosis and developmentzygotemulticellular sporophytezygotehaploid (1n)diploid (2n)Figure 22.16
56 Types of Plants The four principal categories of plants are: bryophytesseedless vascular plantsgymnospermsangiosperms
57 green algae (ancestors) Types of PlantsMossFernsflowering plantsmossesfernsconifersflowersseedsvascular tissueConifersFlowering plantsmulticellularitygreen algae (ancestors)Figure 22.17
58 Types of Plants Bryophytes include mosses. Seedless vascular plants include ferns.Gymnosperms include coniferous trees.Angiosperms include a wide array of plants, such as orchids, oak trees, rice, and cactus.
59 BryophytesBryophytes are close living relatives of the earliest plants that made the transition from living in water to living on land.They lack a fluid transport or vascular system and thus tend to be low lying.
60 BryophytesBryophyte sperm get to eggs primarily by swimming through water, although it’s recently been discovered that tiny animals sometimes transport sperm among mosses.Bryophytes tend to inhabit damp environments.
64 GymnospermsGymnosperms are seed-bearing plants with seeds that are not encased in tissue called fruit.There are only about 700 gymnosperm species, but their presence is considerable, particularly in northern latitudes, where gymnosperm trees, such as pine and spruce, often dominate landscapes.
66 GymnospermsThe sperm of gymnosperms is encased in pollen grains that are carried to female reproductive structures by the wind.Gymnosperms produce seeds in carrying out reproduction.Seeds are structures that include a plant embryo, its food supply, and a tough, protective casing.
68 Gymnosperms male cone Wind carries pollen to female cone. pollen mature treeSperm within the pollen fertilizes one egg within the cone.seedlingEmbryo begins to develop within seed.seedNew pine tree begins to grow.Seed germinates.Figure 22.22
69 AngiospermsAngiosperms, or flowering plants, are seed plants with seeds that are encased in tissue called fruit.Angiosperms are easily the most dominant group of plants on Earth, with some 260,000 species having been identified to date.
70 AngiospermsAngiosperm species include not only plants with flowers, such as roses, but almost all trees except for the conifers, all important food crops, cactus, shrubs, and common grass.
76 EndospermAngiosperm seeds contain tissue called endosperm, which functions as food for the growing embryo.Endosperm supplies much of the food that human beings eat. Rice and wheat grains consist largely of endosperm.
78 FruitAngiosperm seeds are unique in the plant world in being wrapped in a layer of tissue called fruit.Fruit that is edible functions in angiosperm seed dispersal because animals will eat and digest the fruit but then excrete the tough seeds inside, often in a different location.