Plant Kingdom PDF

Chapter 5 Plant Kingdom 1 Generalities Plants are believed to have evolved from common ancestors: ancient green alga. Plants are multicellular photoautotrophic eukaryotes. The plant cells are grouped to form functional tissues and different tissues are grouped to form organs of the plan...

Chapter 5 Plant Kingdom 1 Generalities Plants are believed to have evolved from common ancestors: ancient green alga. Plants are multicellular photoautotrophic eukaryotes. The plant cells are grouped to form functional tissues and different tissues are grouped to form organs of the plant body. Plants are considered as Cormophytes, i.e. possess an erect stalk. They are all multicellular plants; their cells contain vacuoles and are surrounded by cellulosic walls. They are autotrophic and feed by photosynthesis (the main photosynthetic pigments are chlorophyll a and b, as well as carotenoid accessory pigments) and their carbohydrate is stored in the form of starch in chloroplasts and other plastids. Plants are for the majority terrestrial except for some groups, the aquatic plants, that have evolved back with adaptations to the aquatic environments. Figure 61 The phylogeny of the Plant Kingdom 52  Adaptations to land colonization Land colonization was marked by the differentiation of specialized structures for photosynthesis, reproduction, support and anchor into the ground. The increased photosynthetic areas, leaves, are provided with pores allowing gas exchange (oxygen and carbon dioxide). The stems and leaves are covered with a cuticule, preventing water loss. The lignin thickening of the cellulosic walls confers rigidity allowing the support of aerial organs and the upright shape. The differentiation of conducting tissues (xylem and phloem) is essential to transport vital elements (water, minerals and products of photosynthesis) through the plants organs. The roots improve ground mounting, absorption of water and mineral nutrition. Simultaneously, increasingly adapted devices to terrestrial conditions will appear, allowing greater protection of reproductive cells, embryos and spores. The development cycle of plants shows an alternation of two heteromorphic generations: the haploid gametophyte and the diploid sporophyte. Figure 62 Flowering plant organs. 53 Figure 63 Anatomy of a leaf. Plant Kingdom classification Plants can be classified by:  tissue structure: non-vascular (bryophytes) and vascular plants (all others),  reproductive characteristics: non- flowering or flowering plants,  reproducing through spores or seeds,  seed structure: naked or covered seeds inside a fruit. Different classification systems are in common use. In this course, we will use the following classification system of the plant kingdom for simplicity purposes : Figure 64 Representation of the Cormophytes phylogeny 54  Sub-kingdom of nonvascular o Phylum Bryophyta  Class Bryopsida (Mosses)  Class Hepaticopsida (Liverworts)  Class Anthoceropsida (Hornworts)  Sub-kingdom of Vascular o Phylum Pteridophyta (Plants without seeds)  Class Psilotopsida (Whisk ferns)  Class Lycopodiopsida (Club mosses)  Class Sphenopsida (Horsetails)  Class Filicopsida (Ferns) o Phylum Spermatophyta (Seed plants) Subphylum Gymnosperms  Class Coniferopsida (Conifers)  Class Cycadopsida (Cycas)  Class Gingkopsida (Ginkgo)  Class Gnetopsida (Clamydosperms) Subphylum Angiosperms (flowering plants)  Class Magnoliopsida of Dicotyledons (Dicotyledonous, Dicots)  Class Liliopsida of Monocotyledons (Monocotyledonous, Monocots). 55 2 Sub- kingdom of nonvascular plants In many respects, bryophytes are transitional between the green algae (chlorophyte) and the vascular plants. Bryophytes are the first Embryophytes or Archegoniates, gradually adapted to the terrestrial environment. Bryophytes (Greek bruon: mosses) are chlorophyll cormophytes, that means plants with erected stalk or cormus, provided with lateral appendages. The epidermis is deprived of stomata (with some exceptions). Perforations, simple pores, allow the gaz exchange between the atmosphere and the internal tissues of the plant. They have no roots, but rhizoids which mainly provide attachment to the substrate and partially water uptake. They are small plants, without flowers and seeds (cryptogams), and no real conducting tissues or vessels. The development cycle of bryophytes is characterized by an alternation of generations where the gametophyte dominates the sporophyte:  Haploid generation: is the gamete producer corresponding to the gametophyte (vegetative apparatus). Gametes are formed inside the gametangium (plur; gametangia) protected by a wall consisting of one or several cell layers. Male gametangia called antheridia produce mobile flagellated sperm (aquatic fertilization). Female gametangia called archegonia, hence the name given to these plants Archegoniates; produce an egg.  Diploid generation: the sporophyte that corresponds to the spore producer. The sporophyte grows as parasite on the female gametophyte from which it takes the necessary water and nutritive substances. Most Bryophytes form a more or less dense carpet in the under-storey rich in humus. They form the low vegetative cover or moss layer in wet and swampy stations, on trunks, walls and rocks. They are able to withstand long periods of drought by dehydrating and rehydrating with the return of favorable conditions (reviviscence). The phylum Bryophyta includes three classes:  Class Bryopsida (Mosses)  Class Hepaticopsida (Liverworts)  Class Anthoceropsida (Hornworts) 56  Class Bryopsida (Mosses) Mosses are the most common Bryophytes with nearly 14 000 species. Their gametophyte is composed of an upright “stem-like” structure on top of which are found the reproductive organs (gametangia). The axis (cormus) bares leafy blades. They are rudimentary structures with no stomata or veins and are always one cell thick except for the midrib. The leafy blades cells usually contain numerous chloroplasts, except at the midrib. The ground mounting is ensured by multicellular hairs named rhizoids. Figure 65 Female gametophytes holding the sporophytes. The Mosses sporophyte develops on the female gametophyte. The sporophyte’s epidermis shows stomata. It includes: - A haustorial foot which anchors the sporophyte to the female gametophyte from which it absorbs minerals and nutrients. - A seta or stalk holding the capsule. - The capsule. It is covered externally by a membranous or hairy hood called the calyptra. The apex of the sporangium differentiates into a cap-like lid called the operculum, which separate from the rest of the sporangium as cells torn apart. Cells breakage is precise, resulting in one or two rows of complex teeth called peristome teeth. The teeth respond to humidity, bending outward when the air is dry, and spores, therefore, are released. Spore germination is immediate if the conditions are favorable. Each spore gives a branched and chlorophyllous multicellular filament: the protonema (proto: primitive; nema: filament). On the backside of the protonema appear the rhizoids; on its upperside appear buds that will be the starting point for gametophytes. 57  Figure 66 Moss sporophyte’s capsule structure Class of Hepaticopsida (Liverworts) The liverworts are represented by around 6000 species. They grow in very humid environments. Their gametophytes develop directly from spores. There are two groups according to their vegetative structure:  The thalloid liverworts  The leafy liverworts Figure 67 Liverwort germination and growth. 58 2.2.1 Thalloid liverworts (Marchantia polymorpha) The best known species of thalloid liverworts are in the genus Marchantia. The vegetative apparatus (thallus) is represented by a creeping chlorophyllous thallus branched dichotomously and fixed by rhizoids. The thallus, few centimeters thick, is constituted by differentiated cells formingfunctional groups, such as: chlroenchyma (photosynthetic role) on the back or top side, pores and air-carrying chambers (gas exchange), a ventral parenchyma (reserve). The backside bears, in addition to unicellular rhizoids, multicellular scales. Figure 68 Diagram of a cross section in Marchantia thallus. 2.2.2 Leafy liverwort The vegetative apparatus is a creeping axis with dorso-ventral symmetry, with leaf-like structures and rhizoids. “Leaves” of Liverworts, like in mosses, comprise a single layer of undifferentiated cells but do not have a midrib. Many leafy liverworts, like Frullania, have two rows of equal-sized leaves and a third row of smaller leaves along the lower surface of the gametophyte. Figure 69 Leafy liverwort with prostrate branches, Frullania. A: ventral view of a male. B: dorsal view of a female. 59  Class Anthoceropsida (Hornworts) Hornworts include few species (about 300). The gametophyte, in the form of a rosette, bears on its dorsal side the archegonia and the antheridia and on its ventral side unicellular rhizoids that anchor it to the substrate. The sporophyte is composed of a foot (absence of seta unlike the mosses and liverworts) and a long cylindrical capsule. The epidermis of Hornworts sporophyte shows stomata. Several sporophytes can grow on the same gametophyte. The cells of most hornworts have large chloroplast with pyrenoid (like in green algae). Their thalli have pores with underneath cavities and they are not stomata with controlled openings. Figure 70 Anthoceros, a hornowrts the gametophyte holding the sporophyte 60 3 Sub- kingdom of vascular plants Organization of the vegetative organs in vascular plants 3.1.1 Roots In addition to their anchor and absorption roles, roots can sometimes store food and serve in asexual reproduction. Two types of roots are found: taproot (one main principal root and many secondary roots) and fibrous root (numerous primary fine roots). 3.1.2 Stems Functions of the stem are to produce and support new leaves, branches, and flowers; to place them in positions where they can function most efficiently; and to transport materials to and from the roots. Frequently, stems serve to store and transport food, minerals and water (xylem and phloem), carry on photosynthesis, and reproduce new plants. The region of a stem where a leaf or leaves are attached is called nod. The stem region between nodes is called internode. Stems bears (terminal or axillary), branches and flowers. 61 3.1.3 Leaves The leaves intercept light, exchange gases, and provide a site for photosynthesis. Some leaves also store food and water, provide support, or form new plants. A leaf usually has a flattened blade, and in most cases is attached to the twig by the petiole. Within each leaves the vascular tissues form the veins. Each angle between the petiole and the stem is called axil and contains a bud. The arrangement of veins, venation, may be pinnate (one primary vein, and secondary veins branch from the primary vein) or palmate (several primary veins fan out from the base of the blade) or parallel. Phylum Pteridophyta Pteridophytes (pteris: fern) have stems, leaves and roots (rhizophyta, rhizos : roots). They are vascular cryptogams, without flowers or seeds but have conductive tissues (phloem and xylem elements impregnated with lignin). In general, the development cycle of Pteridophytes is very similar to the one of Bryophytes with one important difference: the perennial plant dominating the cycle corresponds to the sporophyte. While the gametophyte or prothallus is small and has a limited life. Like in Bryophytes, the gametophyte is directly derived from the germination of a spore. Like all vascular plants, Pteridophytes have three types of tissues: protective tissues that cover and protect the external surface of the plant ; conductive tissues (Xylem and Phloem), surrounded by the ground tissues. 62 The most primitive Pteridophytes (whisk fern) do not have leaves or roots. More complex groups (club mosses, Horsetail…) have small leaves without petiole and with one vein, called microphylls. For the most evolved (ferns), they have most developed leaves, called megaphylls, or fronds with petiole and limbo with a network of veins and the leaves are inserted directly on rhizomes. The fertile leaves holding the sporangia are called sporophylls. Pteridophytes can be:  Isosporic with identical spores that give one kind of prothallus bearing antheridia and archegonia.  Heterosporic with microspores and megaspores that give two kinds of prothalli. One bears the antheridia and the other the archegonia. The phylum Pteridophyta has expanded greatly since the Paleozoic era (360 million years ago), several groups have disappeared. The current representatives consist of four classes:  Class Psilotopsida (Psilotum or whisk fern)  Class Lycopodiopsida (Club mosses; Lycopodium and Selaginella )  Class Sphenopsida (Equisetum or Horsetail)  Class Filicopsida (Ferns) 63 3.2.1 Class of Psilotopsida (Whisk ferns) The sporophyte is formed of upright green stems, branched dichotomously, attached by a rhizome (underground stem) with rhizoids. The stems are the main photosynthesic organs since they have no “leaves”. They have enations, which are green, leaflike, veinless, photosynthetic flaps of tissue. The sporangia are trilocular (aggregated by three) and are borne on short lateral branches directly on the stems. Psilotopsida are isosporic. Figure 71 Psilotum nudum sporophyte. 3.2.2 Class of Lycopodiopsida The sporophyte is fixed to the ground by underground rhizomes holding roots. The branched stems carry microphylls. The sporophylls bare sporangia grouped in conelike strobili. 3.2.2.1 Lycopodium (Club moss) In Lycopodium the strobili are located at the tips of stems. Lycopodium are isosporic, they have one type of sporophylls and produce one type of spores. 64 3.2.2.2 Selaginella (Spike Moss) Selaginella strobili are formed of microsporophylls where microspores are formed, and macrosporophylls that produce macrospores after meiosis that develop respectively into male and female gametophytes. Figure 73 Slelaginella sporophyte. 3.2.3 Class Sphenopsida (Equisetum or Horsetail) 66 Aerial stems of Equisetum or Horsetail grow from rhizomes bearing roots. The aerial stems have nodes, internodes and ribs. Some species have separate fertile and sterile stems rising from the rhizome. The chlorophyll vegetative stems bear small non photosynthetic leaves, the microphylls, arranged in whorls on the nodes. In some species, lateral branches are inserted at the nodes. Fertile stems are not photosynthetic and have at their end sporangia grouped in strobili. Equisetum is isosporic. The bisexual gametophytes are green and independent. Figure 74 Equisetum sp. Sporophyte 67 3.2.4 Class Filicopsida (Ferns) The sporophyte of Ferns (e.g. Polypodium = Dryopteris) has an underground rhizome. The rhizome carries on its backside adventitious roots. The large leaves or fronds are megaphylls. Ferns are the only vascular cryptogam to have well-developed megaphylls. Commonly, the fronds are compound; that is, the lamina is divided into leaflets, or pinnae, which are attached to the rachis, an extension of the leaf stalk, or petiole. Fronds "emerge" by circinnate vernation, a nearly unique uncoiling process. The tips of young fern fronds are called fiddleheads. Sporangia are grouped in clusters called sori on the back side of the sporophylls. The sorus usually has a protective layer termed indusium (plural: indusia). Most of the ferns are isosporic, others are heterosporic. The gametophyte or prothallus, with a flattened, heart-shaped structure, has many rhizoids. Figure 75 The fern Polypodium sp. structure. 68  Phylum of Spermatophytes The spermatophytes (sperma: seeds) or seed plants are plants with seeds or phanerogams (phaneros: apparent). Seeds replace the spores in the developmental cycle. The male and female gametophytes are very small and develop inside specialized organs in the sporophyte which gives them protection against desiccation.. After fertilization, the ovule becomes a seed. Figure 76 Evolution of the relation gametophyte/sporophyte in plants All spermatophytes possess megaphylls, but they are transformed into needles or scales in some groups. The spermatophytes are divided into two sub-phylum Gymnosperms and Angiosperms. 3.3.1 Sub-phylum of Gymnosperms The name gymnosperm, which literally means “naked seed,” points to one of the principal characteristics of plants belonging to this subphylum—namely, that their ovules and seeds are exposed on the surface of modified leaves known as sporophylls. The ovule is not enclosed in an ovary. The seed is formed from the ovule, but there is no fruit. They lack the specialized water-conducting vessels of flowering plants but have tracheids, Gymnosperms include four classes: Coniferopsida (conifers), Cycadopsida (cycads), Ginkgopsida (maidenhair tree, or ginkgo), and Gnetopsida (gnetophytes). 69 3.3.1.1 Class Cycadopsida (cycads) The plants are fairly large with a short, stout, unbranched trunk. The trunk has a terminal crown of long, leathery leaves giving it the appearance of palm tree. Cycads are dioecious: male and female reproductive organs are found on separate plants. Figure 77 Cycas sp. 3.3.1.2 Class Ginkgopsida Essentially fossils, Ginkgoes are represented by a single extant species, Ginkgo biloba. The veins branch dichotomously, a unique leaf venation pattern. A Ginkgo can always be identified from its fan shaped leaves. Ginkgos are dioecious, with separate male and female plants. Trees can reach a large size (30 meters). Unlike other gymnosperms, Ginkgo biloba loses its leaves in autumn. This tree is often used as ornamental plant in urban areas, because it resists well the air Figure 78 Gingko biloba. pollution and other environmental aggressions. 70 3.3.1.3 Class Gnetopsida They have some characteristics in common with Gymnosperms, and some with Angiosperms. They are unique among Gymnosperm in having vessels in their xylem. Gnetopsida has three genus:  Gnetum: shrubs or vines with large leathery leaves.  Ephedra: much branched shrubs with scaly leaves. Figure 79 Gnetum include climbing plants and Figure 80 Ephedra grow in arid regions, almost everywhere in the world. tropical trees.  Welwitschia: Most of Welwitschia’s body is a long underground taproot. Its short, wide stem forms a shallow disc from which two ribbon-like leaves extend. Figure 81 Welwitschia mirabilis. 71 3.3.1.4 Class Coniferopsida (conifers) Conifers are the largest and most widespread among gymnosperms, e.g. Pine, Fir, Spruce, Larch, Cypress, Redwood (sequoia), Juniper, Yew, etc. They are named conifers because the reproductive organs are grouped inside unisexual cones composed of scales. The leaves are either tough needles or scale-like well adapted to drought. The xylem of conifers is composed of tracheids. Resin canals are found in stems and leaves, hence the appellation of resinous. Most conifers are evergreen (the tree remains green by growing leaves all year long as other leaves fall off) ; only a few are deciduous (lose their leaves at the end of their growing season, usually in autumn). Conifers are generally monoecious (each tree hold both male and female cones) but some species are dioecious. Figure 82 Different type of female cones. 72 Figure 83 Pine tree Figure 84 Pine branch holding both male and female cones 3.3.2 Subphylum of Angiosperms The main feature of angiosperms is the typical flower containing the reproductive organs. Angiosperms are the most diverse group (about 240,000 species), and the most advanced of the Plant Kingdom. They include both herbaceous and woody plants. In Angiosperms (angio: envelope and sperm: seed), the bi- integumented ovule are enclosed in a protective envelop, the carpel, more or less closed in an ovary. After fertilization, the ovary becomes a fruit and the ovule a seed. This group is characterized by a particular mode of fertilization: the double fertilization. Wood is mainly comprising heterogeneous real vessels and fibers. 73 A complete flower consists of four floral whorls inserted on the receptacle. These whorls are from the outside to the inside:  Sepals : mostly green , which together form the calyx  Petals: usually colored, which together constitute the corolla. The calyx and the corolla are the sterile protective parts and do not have a reproductive role. Together they form the perianth.  Stamens: ♂ reproductive parts, which together form the androecium.  Carpels: ♀ reproductive parts, which together form the gynoecium or pistil. Each carpel is formed of a lower part, the ovary containing one or more ovule, an upper part, the stigma which Figure 85 Flower parts receives pollen grain. The pollination (transport of the pollen from the male organ to the female organ) is insured by wind, animals or humans. Angiosperms are divided into two classes: Liliopsida (Monocots, one cotyledon) and Magniliopsida (Dicots, two cotyledons). 3.3.2.1 Class of Monocotyledons Monocotyledons have the following characteristics:  One cotyledon.  Herbaceous plants (except palms...).  Fibrous root system  Number of floral parts by three (or multiple).  Leaves with parallel veins.  Conductive tissues of stems are numerous and scattered. 74 3.3.2.2 Class of Dicotyledons Dicotyledon plants are very diverse. Their main characteristics are:  Two cotyledons.  Herbaceous or woody plants.  Taproot system usually present.  Number of floral parts four or five (or multiple).  Leaves with reticulate veins.  The primary (primary growth) conductive tissues of the stem are arranged in a ring.  Secondary tissues (wood, liber) and cambium. Figure 86 Comparison between monocots and dicots plants. 75