Chapter 1: The Characeae Plant

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10 1.2.6 Species of Nitellopsis Nitellopsis (Fig. 1.11) is a Northern Hemisphere genus, so far absent from Africa, Australia and South America. The genus occurs in relatively deep water in European lakes (Shubert and Blindow 2003), as well as in North America. Nitellopsis obtusa (Desv. in Lois.) J. Groves can be dominant in lakes and frequently co-occurs with the rare charophyte Lychnothamnus barbatus (Pełechaty et al. 2010). It reproduces sexually as usual, but also asexually via distinctive star-shaped bulbils (Fig. 1.11d) sometimes called starch-stars (Fritsch 1948), attached to the lower stems and rhizoids. The capacity for asexual reproduction could improve the success of long-term culturing in the laboratory. The long, ecorticate internodes and branchlets can be large enough for physiological studies (Winter and Kirst 1990; Winter et al. 1999), and it is tolerant of low levels of salinity. The plant body is relatively simple (Fig. 1.11a) with whorls of elongate branchlets and simple bract cells. Stipulodes are absent or rare at the base of the whorl, there is no cortication and few accessory structures (Fig. 1.11c). The oogonia and antheridia are on separate plants (dioecious), and the oogonia can be calcified, with the oospore surrounded by a gyrogonite (Fig. 1.11b). Oospores and gyrogonites of Nitellopsis can be among the largest of extant charophyte reproductive organs. 1.2.7 Species of Nitella suitable for physiological studies Nitella species are identified on the basis of a coronula of 10 cells on the oogonium (Fig. 1.12b) and furcate or forked branchlets, at least in the fertile parts (Fig. 1.12a, c). The terminal branchlet segments or rays are called ‘dactyls’ and the arrangement of cells in the dactyls is used to determine the subspecies and species of Nitella (Fig. 1.4c). The species of Nitella that have been used for physiological studies in the past are generally characterised by long internodes, interrupted by few, sparsely forked whorls of branchlets. Species with short internodes and dense whorls (e.g. Fig. 1.12a) are unlikely to be useful, simply because access to their large cells would be difficult. In general species of Nitella have branchlets in whorls of 6 to 9 (Fig. 1.3c, d, Fig. 1.12c), except where there is more than one whorl of branchlets at the node (a condition termed ‘heteroclemous’ Fig. 1.3d). Branchlets can be once, twice or more times furcate (Fig. 1.4c). Fertile branchlets can be similar to the sterile ones, or contracted into ‘heads’ or ‘spikes’. They can sometimes be covered with a jelly-like substance called ‘mucus’. The reproductive structures occur at the branchlet nodes (forks),
11 where the antheridia are terminal or central in the fork (Fig. 1.12d), and the oogonia are lateral. The taxonomy of Nitella species is gradually being sorted out by various authors, and the overall morphology as well as the characteristics of the oospore are useful clues to identity (Sakayama et al. 2002; Casanova 2009). Species with single-celled dactyls (terminal branchlet segments) (e.g. Fig. 1.12d) are in two groups; Palia and Nitella, those with pluricellulate (multicelluar) dactyls are in subgenus Hyella, and those with two-celled dactyls, of which the terminal one is small, conical and acute (bicellulate e.g. Fig. 1.12c), are in subgenus Tieffallenia. The validity of these higher taxonomic groupings is under investigation, as is the membership of those groups. 1.3 Cellular structures and processes 1.3.1 Cells Characeae organs (internodes, branchlet cells, stipulodes, bract cells etc.) are single cells or groups of single cells. In general they have a typical plant-cell structure with a cellulose cell wall, plasma membrane enclosing a thin layer of cytoplasm containing chloroplasts, mitochondria, nuclei, protein bodies and statoliths, surrounding a large tonoplast-bound vacuole (Fig. 1.13). At the ultrastructural level charophyte cells have characteristics in common with the cells of other algae (Zygnematophytes), mosses, liverworts and vascular plants (Casanova 2007). The plant axis is produced via sequential divisions of a dome-shaped apical cell, the products of which differentiate into discoid nodes and elongate internodes (Fritsch 1948). Axis nodal initials divide into central cells and a series of peripheral cells that differentiate into branchlet, stipulode, cortical and gametangial initials (Bharathan and Sundralingham 1984). Branchlet initials divide further into branchlet internode cells and nodal complexes (Fig. 1.13). The cortical filaments, bract cells and gametangial initials develop from the branchlet nodes. The cell wall construction in charophytes is basically fibrillar, crystaline cellulose (Casanova 2007), sometimes with a superficial layer of mucilage (check Mary). The chloroplasts are lodged in the peripheral layer of the cytoplasm, close to the cell wall, and are arranged in longitudinal lines. Internode cells contain multiple copies of the nucleus formed via amitosis (Fritsch 1948). The axial node complex consists of different numbers and arrangements of cells and can be used to distinguish among genera: in Chara there are two central cells in the nodal complex, and in Nitella, Tolypella and Lamprothamnium one of the central cells undergoes further division into daughter cells (Frame and Sawa 1975).
Page 1 and 2: 1 Chapter 1: The Characeae Plant 1
Page 3 and 4: 3 1.1 General Morphology The chara
Page 5 and 6: 5 largest cells, and for this reas
Page 7 and 8: 7 Lamprothamnium in 1916 to avoid
Page 9: 9 of inflated branchlets develop,
Page 13 and 14: 13 zygote and the maternal spiral
Page 15 and 16: 15 1.4.1 Systematics and the conce
Page 17 and 18: 17 definition of species (Proctor
Page 19 and 20: 19 bulbilifera. The natural range
Page 21 and 22: 21 do they occur?’ are central t
Page 23 and 24: 23 of animal and algal epiphytes (
Page 25 and 26: 25 1.5.4.4 Temperature Storage tem
Page 27 and 28: 27 annuals (i.e. reproducing once
Page 29 and 30: 29 growth has taken place (e.g. Ch
Page 31 and 32: 31 Figure 1.15 The characeae oospo
Page 33 and 34: 33 antheridia clustered at the bas
Page 35 and 36: 35 Casanova, M.T. and Karol, K.G.
Page 37 and 38: 37 Proctor, V.W. 1960. Dormancy an

Chapter 1: The Characeae Plant

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