Diversity of Seed Plants and their Systematics Gymnosperms I General Features of Gymnosperms and their Classification Dr. NUPUR BHOWMIK Department of Botany University of Allahabad Senate Hall Allahabad – 211002 [email protected] Date of submission: 27/06/2006 Keywords Coniferophyta, nucellus, microsporophylls, ovule, archegonium, Pteridosperms, epiphyte, parasite, prothallial cells, integument, dendrochronological, Cretaceous, Devonian. GENERAL FEATURES OF GYMNOSPERMS AND THEIR CLASSIFICATION Gymnosperms are seed plants that compose a major component of the temperate forests of the world. They are widely appreciated for the great range of variation in habit and shapes. They have some of the tallest trees known in the plant kingdom. Sequoia sempervirens, commonly called the Californian or "Coast Redwood" because of its red coloured bark and red heart wood, reaches a height of 112 metres, and a diameter of 5 metres(See Pl. 1 & Pl. 2). Another species, Sequoiadendron giganteum from California, U.S.A. commonly called "Big Tree" or the Wawona Tree, is a little shorter in length about (87m) but attains a diameter of more than 10 metres (See Pl. 3). The age of tallest tree is estimated from 400 to 800 years. The oldest redwood on record, which has a diameter of 3.7 metres across is estimated to be 2,200 years. However, the distinction of being the oldest living tree on earth has now gone to a species of Pinus, P. aristata now called as P. longavea. A tree of this species is growing at 3275 m. above sea level in east Nevada-Utah State boundary(See Pl. 4). The age has been dendrochronologically shown to be 4900 years old (Foster & Gifford, 1974). Other estimates of age of the oldest tree of this species are mentioned as their being as old as 6000 years old (Sahni, 1990). However, all gymnosperms are not tall and gigantic. There are some small herbaceous looking forms also, like Zamia pygmaea and Stangeria paradoxa(See Pl. 5) or the two parasitic forms, Parasitaxus (= Podocarpus) ustus(See Pl. 12) and Gnetum trinerve. While the former parasitises on Dacrydium taxoides, the latter is a parasite in Cinchona forests of Chimboraza (Pearson, 1929). A species of Zamia, Z. pseudoparasitica is exceptional among all gymnosperms in being epiphytic(See Pl. 6). A few gymnosperms may be lianas or climbers. They are mostly evergreen, but some such as Larix and Taxodium are deciduous(See Pl. 7). The plants are mostly xerophytic but plants such as Taxodium are also known to grow in swamps or marshy places in Mexico and Florida. (See Pl. 8) The gymnosperms include ancient lines of seed-bearing plants whose evolutionary history dates back to the Upper Devonian, roughly about 375 million years ago. One of the extinct gymnosperms consisted of an assemblage of plants whose foliage was fern-like in general appearance yet it possessed a primitive type of seed. This group of plants was named "seed ferns" or "pteridosperms". Undoubted gymnosperms had come into existence during the Lower Carboniferous (about 355 million years back) and had reigned over the plant kingdom by virtue of their dominance right upto the Cenomanian (about 96 million years from now) till the advent of angiosperms at the end of the Cretaceous. The surviving gymnosperms no longer dominate the plant kingdom but can still claim to own about 70 genera and 800 species distributed in various parts of the world. They include the newly described genera Chigua and Wollemia. While Chigua belonging to Zamiaceae of cycads was described by D.W. Stevenson in 1990(See Pl. 9), the genus Wollemia assigned to Araucariaceae of conifers was discovered a few years later by Hills et al. (1996) (See Pl. 10). The largest group is the Coniferophyta, which includes pines, firs, cedars, spruces and junipers which form extensive forests of northern and southern hemispheres. It contrast, living representatives of the Ginkgophyta and Cycadophyta are veritable "living fossils", the former being represented by the sole living member Ginkgo biloba (See Pl. 11) existing in wild state in certain mountains in southeastern China and the latter are relicts from the past age being confined to limited areas in tropics and sub-tropics. The last group of living gymnosperms is the Gnetophyta represented by Welwitschia, Ephedra and Gnetum. Gymnosperms are highly valued for their timber. They are also sources of starch, resins, essential oils, drugs, edible nuts etc. Many are cultivated as ornamental plants ("palms") and are found in gardens all over the world while some conifers are sold as Christmas trees. Some gymnosperms like the Ginkgo are held in high esteem and are found growing around Buddhist temples in China and Japan. Gymnosperms (gymnos = naked, sperma = seed) include plants whose ovules are naked and freely exposed for pollination. They are borne on microsporophylls, scales, or comparable structures. In angiosperms (angios = closed, sperma = seed) ovules are enclosed in a carpel and usually completely closed at the time of pollination. The sporophyte is usually arborescent comprising of large or small woody trees or shrubs. Few may be lianas or climbers. Most gymnosperms are evergreen but some are deciduous such as Larix and Taxodium(See Pl.7). The plants are mostly xerophytic. Gymnosperms show the following features: 1. The plants have a long lasting tap root system. The vascular cylinder is di-to polyarch, xylem exarch. Main elements of xylem are tracheids. Phloem is composed of sieve cells and lacks companion cells. Some forms exhibit additional symbiotic relationship between roots and algae in coralloid roots (Cycas) and between roots and fungi in mycorrhizic roots (Pinus). 2. The sporophyte shows unlimited growth of aerial trunk by means of apical and lateral meristem which produce secondary vascular tissues. 3. The aerial trunk is branched or unbranched and woody. Majority of gymnosperms have branched stem (except in Cycas and Zamia). In Pinus branches are of two types :- (a) long shoots or branches of unlimited growth (b) dwarf shoots or branches of limited growth bearing clusters of variable number of needle shaped leaves and collectively called as spur. Vascular bundle of stems are collateral, endarch or mesarch, open and arranged in a ring. 4. Secondary growth very pronounced. Secondary vascular tissues consisting mainly of tracheids and sieve cells. Normally xylem lacks vessel elements and the phloem lacks companion cells. But both vessels and companion cells present in members of Gnetales. Mature metaxylem and secondary xylem elements have bordered pits of various types. Wood or secondary xylem of two types namely, manoxylic and pycnoxylic. In manoxylic type (as in Cycas) wood is less, porous and soft. There is a large cortex and pith and parenchymatous rays are wide. In pycnoxylic type (as in Pinus) wood is dense or compact with small xylem rays and reduced pith and cortex. 5. Leaves are diverse in form and arrangement. They are both simple and compound ranging in size from a minute scale to leaves a few meters long. Arrangements of leaves usually spiral, they may also be whorled as in Cedrus or opposite decussate as in Cupressaceae, Welwitshchia(See Pl. 12) and Gnetum. Scale leaves are microphyllous whereas larger leaves are megaphyllous and their vascular supply always leaves a leaf gap in the stem stele. Venation may be parallel (Agathis and Welwitschia), reticulate (Gnetum) or dichotomous (Ginkgo) or as in most genera there may be a single vein. Leaves may be dorsiventral or isobilateral, amphistomatic or hypostomatic with sunken stomata. Mesophyll may be differentiated into spongy and palisade parenchyma (Cycas, Gnetum) or undifferentiated (Pinus). Transfusion tissue is a prominent feature. Leaf surface protected by a thick cuticle and sometimes by an additional waxy layer. Resin canals occur in all conifers (except Taxus) and mucilage ducts in cycads and Ginkgo and latex tubes in Gnetum. Vasculature of petiole quite variable. In Cycas, vascular bundles are arranged in a horse-shoe shaped manner or look like an inverted omega (U). They are diploxylic having both centripetal and centrifugal xylem. In Ginkgo petiole receives two vascular bundles and each half of lamina is supplied by one bundle which later dichotomizes. In Gnetum, the petiole shows an arc-shaped arrangement. 6. Vegetative methods of reproduction are rare in gymnosperms but vegetatively propagating bulbils are known in Cycas. 7. Plants are heterosporous. They may be monoecious (Pinus) or dioecious (Cycas). Reproductive structures are borne in cones or strobili that are either staminate (male) or ovulate (female) except in Cycas where ovules are borne on loose megasporophylls. Sporangia are borne on fertile leaves or leaf-like structures called microsporophylls (in male cone) and megasporophylla (in female cones) which are arranged spirally around a central axis. 8. Microsporangia are borne on abaxial or lower surface of microsporophylls. They may be numerous and grouped in sori (Cycas) or reduced to two (Pinus). Microspores are produced in tetrads. 9. Megasporangium or ovule is borne on adaxial or upper surface of megasporophyll or ovuliferous scale and are generally orthotropous. Young megasporangium consists of a nucellus which is surrounded by a sheath or integument and inside the nucellus is single functional megaspore. A narrow passage above the nucellus in the integument is the micropyle. 10. Microspore and megaspore germination in situ, producing micro-and megagametophytes which are not autotrophic. Microspores have partly developed endosporic male gametophytes when they are transferred to micropyle of ovule. The microspore or pollen grain at the time of shedding may have only one prothallial cell (Cycas) or two prothallial cells (Pinus), the former being liberated at three-celled stage and latter at four-celled stage. 11. The microspores or pollen grains are borne by wind and enter the ovule directly through the micropylar canal. The micropyle in almost all gymnosperms secretes a sugary exudate called the "pollination drop" which not only receives the pollen grains but also transports them to nucellus of ovule.