Chapter 1: The Characeae Plant

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12 The contents of the cell are in continual movement in healthy internodal cells, with active streaming clearly visible under relatively low magnification (10-20 x magnification). The lines of chloroplasts in the peripheral cytoplasm are interrupted by a less-dense part of the cytoplasm, around which the cytoplasm rotates (Fritsch 1948). The vacuole that takes up the majority of the cell volume in the centre of elongate characeae cells is responsible for generating and maintaining cell turgor (the internal pressure that maintains the rigidity and shape of the cell) (Pickett-Heaps 1975). 1.3.2 Gametangia The charophyte plant body is haploid (i.e. n chromosomes) and gametes (also n chromosomes) are produced via mitoitic division in the reproductive organs. Characeae male gametes consist of motile sperm cells (n chromosomes) (Fig. 1.14a) that swim through the water to fertilise the female egg cell (n chromosomes). Specialised, diploid (2n), seed-like oospores (Fig. 1.1c) are the product of fertilisation of the egg cell by a sperm cell. Characeae gametangia, or reproductive organs, have a distinctive morphology (Fig. 1.5). Unlike other algae, the gamete-producing cells (spermatogenous threads (Fig. 1.14b) and egg cell) are surrounded by sterile cells that protect them. These organs are variously called (for male organs) the globule or antheridium; and (for female organs) the nuclule, oogonium or oosporangium. Development of the oogonium progresses from a single peripheral cell on the branchlet node, producing the five spiral cells and an oosphere mother cell. The spiral cells elongate around the mother cell which divides to produce the oosphere or egg cell (Leitch et al. 1990). The oogonium has one or more basal cells. The spiral cells divide to produce a crown-like structure (coronula) of either five cells (in Chara, Lamprothamnium, Nitellopsis and Lychnothamnus) or ten cells (in Nitella and Tolypella). Antheridial development similarly proceeds from a peripheral cell (Pickett-Heaps 1968), and the mature antheridium consists of a mass of spermatogenous filaments or threads attached to manubria cells via capitula cells, surrounded by (usually) eight shield cells (Fig. 1.14b). When the male gametes (spermatids (Pickett-Heaps 1968b): Fig. 1.14a) are mature they exit the spermatogenous threads via pores (Fritsch 1948), and the shield cells separate slightly to release them, eventually falling apart altogether (Fig. 1.14c). When the egg cell is ready to be fertilised the cells below the coronula become swollen and gaps are formed between the spiral cells allowing entry of the sperm cells. After fertilisation the egg cell becomes the diploid zygote (i.e. 2n chromosomes) and a complex, multilayered wall is formed by both the developing
13 zygote and the maternal spiral cells (Leitch et al. 1990). This specialised zygote is called the oospore and its multilayered wall allows it to resist desiccation and destruction during dispersal and dormancy. 1.3.3 The oospore Characeae oospores are distinctive and easy to recognise (Fig. 1.1c). In all the genera except Nitella there is sometimes the development of a gyrogonite, or calcified ‘shell’ around the oospore, deposited during the last stages of development on the parent plant (Fig. 1.15a). Gyrogonites can gradually decompose over time, or break open when the oospore germinates. The oospore wall, within the gyrogonite, is formed by both the egg cell (internal layers) and the oogonium cells (external layers), and is characterised by spiral sutures or lines on its surface (Leitch 1989). The flat part of the oospore wall is called the fossa, and the sutures are called the striae. Sometimes the striae have flanges (where the oospore wall is formed on the side walls of the spiral cells; Fig. 1.15b), and sometimes a thinner structure called a ‘ribbon’ (John et al. 1989) or thicker structures (pachygyra; Fig. 1.15c) are formed. The fossa can be variously ornamented with grains (typically so for Lamprothamnium) or fibres. The ornamentation becomes amazingly diverse in the genus Nitella, with verrucae, tuberculae, reticulae and every combination of wall construction imaginable (Fig. 1.15d, e, f) (John and Moore 1987). The base of the oospore is impressed with the shape and arrangement of the basal cell complex. In Chara there is a single basal cell (Fig. 1.15g), in other genera there can be more (Fig. 1.15h). The apex of the oospore exhibits a ‘w-shaped’ suture where the five spiral cells join up (Fig. 1.15i). Oospores can be different colours, ranging from pale yellow to black. In some Nitella species oospore colour is an indication of the maturity of the oospore, and degree of development of the oospore wall (Casanova 1991). Sometimes oospore colour is useful for distinguishing species of Chara (Zaneveld 1940). Oospores are densely packed with starch grains (amyoplasts), the primary reserve for the germinating plant. Healthy oospores are turgid and resistant to external forces. They can be handled easily with fine forceps. The distinctive differences among charophyte oospores allows them to be used in identification of different charophyte species (Haas 1994, Sakayama et al. 2002, Casanova 2005, Casanova 2009). Cultures of charophytes for physiological studies can be started by collecting mature oospores, sterilising their surface and germinating them under controlled or known conditions (see section 1.5.5).
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 and 10: 9 of inflated branchlets develop,
Page 11: 11 where the antheridia are termin
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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