Chapter 1: The Characeae Plant

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2 1.5.4.6 Depth 1.5.5 Culturing characeae 1.5.6 Life History 1.5.7 Population ecology 1.5.8 Application of ecological knowledge Abstract The aim of this chapter is to give physiologists a thorough grounding in the morphology, taxonomy and ecology of the characeae plant. The morphology of characeae is depicted and explained, with specific examples of the morphological characteristics of different species or species groups that are used in physiological studies. The details of characeae cellular structure in growing plants and in the reproductive organs are reviewed. The history of characeae taxonomy and nomenclature is outlined, along with the most recent approaches to charophyte systematics (and what name to use for characeae e plants in physiological studies), and finally the patterns of characeae distribution and requirements for characeae growth in natural situations are explained, and related to the culture and growth of characeae for physiological studies. Michelle T. Casanova Centre for Environmental Management, University of Ballarat, Mt Helen, Vic. 3350, Australia and Royal Botanic Gardens, Melbourne. Birdwood Ave, South Yarra, Victoria 3141, Australia Current Address: 273 Casanova Rd, Westmere Vic 3351 Australia Email: amcnova@netconnect.com.au
3 1.1 General Morphology The characeae thallus (or plant body) is similar in appearance and size to the plant body of other submerged plants such as Ceratophyllum or Myriophyllum. Charophytes consist of long photosynthetic stem-like structures (axes) anchored in the soil, with whorls of leaf-like organs (branchlets) along the stem (Fig. 1.1a). Close examination reveals that the structure of characeae is very different to that of flowering plants. Instead of roots they have colourless rhizoids, instead of leaves they have whorls of branchlets of limited growth, instead of stems they have an axis of giant cells joined end-on-end, and instead of flowers and fruit they have relatively simple reproductive structures (the oogonium and antheridium, Fig. 1.1b) that produce gametes. The product of fertilisation of the gametes is an oospore (Fig. 1.1c) rather than a seed. The thallus of characeae is essentially filamentous. The axes (stems) are made up of long, multinucleate, single cells interrupted by multicellular nodes (Fig. 1.2). There is no development of tissues such as parenchyma in characeae, although the axial nodes approach such an arrangement. Several organs of limited growth (branchlets, stipulodes, cortical filaments) arise in whorls at the nodes (Fig. 1.2). Branchlets are the ‘leaf-like’ organs that occur in spreading whorls, and below these there are often whorls of smaller cells called stipulodes. In many species of Chara the stipulodes occur in two whorls, the upper whorl pointing upwards, the lower whorl pointing downwards (Fig. 1.2a, b). In the genus Lamprothamnium and some species of Chara the axial node can also be the site of gametangial development (Fig. 1.2c). Branchlet arrangement (Fig. 1.3) and morphology (Fig. 1.4) varies among the genera, but is characterised by elongate multinucleate cells interrupted by multicelluar branchlet nodes. Other cellular structures can be produced at the branchlet nodes, namely bract cells (Fig. 1.3a, b, Fig. 1.4a, b), secondary and tertiary (et seq.) branchlet segments or rays (Fig. 1.3c, d, Fig. 1.4c, d), cortical filaments (Fig. 1.4a) and gametangial initials (Fig. 1.4). Some species of Chara have elongate bract cells in whorls (verticillate) at the branchlet nodes, as well as at the apicies of the branchlets (Fig. 1.3a). Other species produce unilateral bract cells (Fig. 1.4a). Lamprothamnium exhibits the same overall branchlet morphology as Chara but the bract cells are generally inserted at angles of 45° to 90° to the branchlet, forming a ‘cage-like’ structure around the nodal complexes (Fig. 1.3b, Fig. 1.4b). Nitella species have branchlets that are divided or forked (furcate) into separate rays or segments (Fig. 1.4c), which can be
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Chapter 1: The Characeae Plant

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