seed embryo microscopic gametophytes Gymnospermes Angiosperms

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1 Gymnosperms

2 Spermatophytes It is the group that includes seed-producing plants. The seed contains embryo, protects it during adverse conditions and contains all nutrients needed for the seedling development. They show microscopic gametophytes. They do not need anymore of water for reproduction. Male gametophyte (polle grain) is released at maturity and dispersed by wind, insects or animals. They comprises Gymnospermes (plants with ovule/seed naked) and Angiosperms (flowering plants with ovule/seed protected by ovary).

3 Gymnosperms They are seed plants in which the ovules are carried naked on the cone scales, in contrast to the angiosperms, in which they are enclosed by an ovary. Gymnosperms date from the Carboniferous ( millions years ago) and subsequently they dominated the floras of the world until the Cretaceous (65 millions years ago), since when they have been progressively displaced by the angiosperms (flowering plants). Gymnosperm is used only informally to describe what are now classified as 4 separate divisions: Cycadophyta (100 species), Ginkgophyta (1 living fossil species), Coniferophyta (550 species), Gnetophyta (70 species). Although they comprises only 720 species, they form the dominant vegetation in large areas of the world (e.g. taiga = forest of conifers). Numerous species are used for pharmaceutical purposes, notably conifers for the preparation of resin derivatives employed in the treatment of respiratory disorders.

4 Gymnosperms They are heterosporous plants, bearing megasporangia (ovules) producing megaspore retained inside the sporangium (endosporic development); they give rise to the female gametophyte, and microsporangia producing microspores that give rise to the male gametophyte (pollen grain). Ovule is composed of a central mass of tissue (nucellus), surrounded by 1 or 2 protective layers (integuments), which eventually give rise to the seed coat. Within the nucellus is a large structure, the embryo sac, which has developed from the megaspore and contains the naked egg cell. Nucellus bears a canal in the coverings called micropyle through which the pollen tube usually passes during fertilization; later, when the seed matures and starts to germinate, the micropyle serves as a minute pore through which water enters. Seed is a fertilized ovule, composed of integuments, embryo and nutritive substances. It replaces spore as diffusion unit. Fertilization is not more linked to the presence of water: male gametophyte (pollen grain), transported to the female gametophyte inside the ovule, emits the pollen tube releasing the male gamete (without flagella) directly to the female gamete.

5 Gymnosperms Pollination is the transfer of pollen grains from male to female gametophytes or from micro- to macro-sporangium. This process facilitates contact between male gametes and the female ovum, leading to fertilization, development of seed, and thence a new plant. Gymnosperms have anemophilous pollination. In gymnosperms, the pollen tube developing from a pollen grain grows down and penetrates the neck of the archegonium, facilitating contact between the sperm cells (male gametes) and the ova (female gametes). Most archegonia contain many ova, so that multiple fertilization can occur, although only 1 sperm can fertilize an egg cell. Unlike angiosperms, which do not possess archegonia, pollen cones and seed cones mature at different times within a season, so that there is usually a long interval between pollination and fertilization. Gymnosperm have an alternation of 2 heteromorphic generations with large sporophytes (dominant) and microscopic, dipendent gametophytes (only 4 cells in the male gametophyte).

6 Gymnosperms consist of tree or shrubs, anyway woody plants possessing secondary thickening by vascular and cork cambium. They have homoxil wood with only tracheids having conducting and supporting functions.

9 briophytes and pteridophytes spermatophytes (gymnosperms and angiosperms)

10 Relations between gametophyte and sporophyte

The megaspore gives rise to the femal gametophyte, containing nucellus with one or more archegonia; so,

11 ovule female gametophyte Ovule is a megasporangium containing a megasporocyte that undergoes meiosis giving 4 haploid cells: one is the megaspore, while the other 3 degenerate. The megaspore gives rise to the femal gametophyte, containing nucellus with one or more archegonia; so, more than one egg cell may be fertilized, but only one embryo may survive. Nucellus is composed of a nutritive tissue feeding embryo during ontogeny.

12 Different stages leading to the fertilization of the egg cell. In gymnosperms, pollination is known as micropylar, because the pollen grain attaches directly to this canal of the naked ovule. Female gametophyte and so embryo develop inside parent sporophyte (endosporic development).

13 Ovules and seeds are unprotected and beared on the external surface of specialized leaves known as megasporophylls that in conifers are grouped in cones or strobila.

14 Cones are groups of closely packed sporophylls arranged around a central axis.

Each microspore germinates to form the male gametophyte, a structure made up of winged pollen grains (for wind pollination) containing 2 protallial cells, one

15 Each microspore germinates to form the male gametophyte, a structure made up of winged pollen grains (for wind pollination) containing 2 protallial cells, one generative cell (forming 2 sperms) and one tube cell (generating the pollen tube). In gymnosperms there are not antheridia (water is no more needed for reproduction).

16 The pollen grain contains 3 haploid nuclei (a tube nucleus and 2 sperm nuclei), which pass down the pollen tube to the ovum. One of the sperm nuclei fertilizes the ovum, and the second degenerates as well as the tube nucleus.

17 Seed of conifers is composed of 2 diploid generations (the outer integuments and the embryo) and one haploid generation (food store tissue). Embryo is composed of an hypocotyl-root axis with a root cap protecting root merystems and an apical meristem to the endings. Gymnosperm have usually numerous cotyledons.

18 Life cyle of a pine

19 CONIFEROPHYTA The biggest division of gymnosperm, with a long fossil history, comprising trees (e.g. sequoia) and shrubs (e.g. juniper), nearly all of which are evergreen, commonly with monopodial crowns. The leaves are often needle- or scale-like. Branches with short (brachyblasts) and long shoot (macroblasts). The wood (homoxil) lacks vessels (trachea); only tracheids with areolate pits. Most conifers are resinous due to the presence of resin canals. Fertile parts occur in unisexual cones, variously containing sterile scales. The ovule and seed are naked and borne on a scale. Are important for timber and paper production. There are about 550 extant species.

20 CONIFEROPHYTA

21 CONIFEROPHYTA branch of pine with mature and young ovuled cones

22 CONIFEROPHYTA branch of pine with male cones (bearing microsporangia)

23 CONIFEROPHYTA ovuled cones male cones

24 CONIFEROPHYTA ovuled cones of fir

25 CONIFEROPHYTA ovuled cones of larch

26 CONIFEROPHYTA ovuled cones of cypress

27 CONIFEROPHYTA ovuled cones of juniper

28 CONIFEROPHYTA t.s. of a pine leaf

29 CONIFEROPHYTA t.s. of pine stem (secondary thickening) The wood of conifers is composed only of tracheids (homoxil wood), while phloem is composed only of sieve cells.

30 CONIFEROPHYTA Yew seeds surrounded by arils (no cones) that aid seed dispersal by providing food as an attractant and reward to the dispersers. Yew is a dioecious plant, with male and ovuled cones on separate individuals.

31 CONIFEROPHYTA Giant sequoia of California; it can reach about 80 m of height and a weight of 1800 tons.

32 CYCADOPHYTA Cycaodophyta is a division of gymnosperms comprising plants with leaves and habit similar to those of palm trees, although some species are quite small. Cycads are dioecious, and most bear large, coloured, female or male cones. Pollen grains have motile spermatozoa within them, which is a very primitive feature. Cycads have enthomophilic pollination. Formerly they were much more important and, following their appearance in the Permian ( millions years ago), remained important members of the world s Mesozoic floras (up to 65 millions years ago). Their reduction as marked in the late Cretaceous as they were progressively displaced by angiosperm trees. There are 9 or 10 genera, and about 100 extant species. All are tropical or subtropical.

33 CYCADOPHYTA Ovuled cones of an African Cycad seeds of Cycas

34 CYCADOPHYTA 4 genera are American; 5 are Old World (Australian and south-eastern African)

35 GINKGOPHYTA The division of gymnosperms that includes only the extant Ginkgo biloba (maidenhair tree) and its extinct relatives. They first occurred in Triassic ( millions years ago) and in the subsequent Jurassic Period ( millions years ago) their distribution was practically worldwide. The surviving species is restricted in the wild to China, and its leaves are strikingly similar to fossil Ginkgo leaves from the Triassic. The restricted geographical range, the unchanged appearance of the leaves, and the motile male sperms have together led to the maidenhair being referred to as a living fossil.

36 GINKGOPHYTA Gynkgo biloba is a dioecious, deciduous tree native to China, bearing fan-shaped leaves with open dichotomous venation. The naked seed is oily and edible but the embryo is bitter. It is widely cultivated.

37 primitive fertilization occurring in Ginkgo and Cycas flagella

38 GNETOPHYTA A remarkable and probably artificial division of gymnosperms that comprises only the Gnetales. There are 3 constituent genera, embracing trees, shrubs, and lianes (Ephedra and Gnetum), and even turnip-like plants (Welwitschia). They show some features being referred to as the closest living relative of angiosperms. These are: strobyla similar to inflorescences; ovule within the perianth tube of flower-like structures; absence of archegonia; tentative of double fertilization (without the formation of endosperm); enthomophilic pollination (production of nectars); presence of trachea into xylem; leaves in some cases similar to those of angiosperms.

trees. They are dioecious, and often cauliflorous.

39 GNETOPHYTA The sole genus, with 28 species, of the pantropical gymnosperm family Gnetaceae, of which 26 species are woody climbers with gouty nodes, and 2 are trees. They are dioecious, and often cauliflorous. The seeds are naked, borne on racemes, and edible in many species. hard leaves of Gnetum, similar to those of Angiospems

Their ovule is naked within the perianth, which becomes woody around the

40 GNETOPHYTA a b Ephedra nebrodensis Ephedra sinica Ephedra is a genus of shrubs which have scalelike leaves and whip-like, slender green stems. Their ovule is naked within the perianth, which becomes woody around the seed. There are 40 species, found in the warm temperate regions of each hemisphere.

GNETOPHYTA Welwitschia, the sole genus, with only one species (W. mirabilis) of the remarkable gymnosperm family Welwitschiaceae of the deserts of southwestern Africa.

41 GNETOPHYTA Welwitschia, the sole genus, with only one species (W. mirabilis) of the remarkable gymnosperm family Welwitschiaceae of the deserts of southwestern Africa. It has a barrel-like stem with a deep tap root, bearing on its rim big, tattering, strap-like leaves, and ovules within the perianth tube of the female flower-like structure.

Botany: An introduction to plant biology, 5 ed. Mauseth. Chapter 22

63 UNIT 5: GYMNOSPERMS th Botany: An introduction to plant biology, 5 ed. Mauseth. Chapter 22 OBJECTIVES This lab considers the gymnosperms, vascular plants that produce seeds but lack flowers and fruits.