The green algae have always attracted a lot of interest because, as a group, they share a good deal in common with the higher plants in terms of ultrastructure, metabolism and photosynthetic pigments, pointing to the likelihood of a common ancestor. They possess both chlorophyll a and b and certain carotenoids, store carbohydrate in the form of starch, and generally have a rigid cell wall containing cellulose. The starch is stored in structures called pyrenoids, which are found within chloroplasts. There are two phylogenetically distinct lines of green algae, the Charophyta and the Chlorophyta; the latter are much the bigger group, but the charophytes seem to be more closely related to green plants (see Figure 9.18).
Chlorophytes demonstrate a wide variety of body forms, ranging from unicellular types to colonial, filamentous, membranous and tubular forms. The vast majority of species are freshwater aquatic, but a few marine and pseudoterrestrial representatives exist.
The genus usually chosen to illustrate the unicellular condition in chlorophytes is Chlamydomonas (Figure 9.5). This has a single chloroplast, similar in structure andshape to that of a higher plant, and containing a pyrenoid. Situated together at the anterior end is a pair of smooth or whiplash flagella, whose regular, ordered contractions propel it through the water. A further structural feature found in Chlamydomonas and other motile forms of green algae is the stigma or eye-spot; this is made up of granules of a carotenoid pigment and is at least partially responsible for orienting the cell with respect to light.
Reproduction in Chlamydomonas and other unicellular types under favourable con-ditions of light, temperature and nutrients, occurs asexually by the production of zoospores. A single haploid adult loses its flagella and undergoes mitosis to produceseveral daughter cells, which then secrete cell walls and flagella and take up an in-dependent existence of their own. This can result in a tremendous increase in num-bers; a single cell can divide as many as eight times in one day. Sexual reproduc-tion in Chlamydomonas, which occurs when conditions are less favourable, differs in detail according to the species (Figure 9.5).
Any one of the three variants of ga-mete production seen in the algae may be seen: isogamy, anisogamy and oogamy. In all cases, two haploid gametes undergo a fusion of both cytoplasm and nuclei to give a diploid zygote. The gametes may simply be unmodified haploid adult cells, or they may arise through mitotic cleavage of the adult, depending on the species. The process of isogamy, where the two gametes are morphologically alike and cannot be differentiated visually, only occurs in relatively lowly organisms such as Chlamydomonas. In some species we see the beginnings of sexual differentiation – there are two mating strains, designated + and −, and fusion will only take place between individuals of opposite strains. The diploid zygote, once formed, often develops into a tough-walled protective spore called a zygospore, which tides the organism over conditions of cold or drought. At an appropriate time the zygospore is stimulated to recommence the life-cycle, and meiosis occurs, to produce haploid cells, which then mature into adult individuals.
In C. braunii, sexual reproduction is anisogamous; a + strain produces eight mi-crogametes and a − strain produces four macrogametes. In C. coccifera, simple oogamy occurs, in which a vegetative cell loses its flagella, rounds off and enlarges; this acts as the female gamete or ovum, and is fertilised by male gametes formed by other cells.
The next level of organization in the green algae is seen in the colonial types, typified by Volvox. These, like the unicellular types, are motile by means of flagella, and exist as a number of cells embedded in a jelly-like matrix (Figure 9.6). Both the number of cells and the way they are arranged is fixed and characteristic of a particular species. During growth, the number of cells does not increase. In simpler types, all cells seem to be identical but in more complex forms there are distinct anterior and posterior ends, with the stigma more prominent at the anterior, and the posterior cells becoming larger. Reproduction can occur asexually or sexually.
The diversity of body forms in multicellular chlorophytes referred to earlier is matched by that of their life cycles. Two examples are described here.
Oedogonium is a filamentous type. When young it attaches to the substratum bya basal holdfast, but unless it lives in flowing water the adult form is free-floating. Asexual reproduction occurs by means of motile zoospores, which swim free for around an hour before becoming fixed to a substratum and developing into a new filament. In sexual reproduction, the process of sexual differentiation is carried a step further than we have seen so far, with two separate filaments producing gametes from specialised cells called gametangia (Figure 9.7). These are morphologically distinct, with the male being termed an antheridium and the female an oogonium. Gametes (morphologically
distinct: anisogamy) fuse to form a resistant zygote or oospore, which, when conditions are favourable, undergoes meiosis to produce four haploid zoospores, each of which can germinate into a young haploid filament. The oospore is thus the only diploid phase in the life cycle. In Oedogonium there are species with separate male and female filaments (dioecious) as well as ones with both sexes on the same filament (monoecious).
A second main form of multicellularity in green algae is the parenchymatous state, by which we mean that the cells divide in more than one plane, giving the plant thickness as well as length and width. An example of this is Ulva, the sea lettuce, a familiar sight at the seaside in shallow water, attached to rocks or other objects. Ulva has a flat, membranous structure, comprising two layers of cells. Reproductively it is of interest because it features alternation of generations, a feature of all the higher green plants. This means that both haploid and diploid mature individuals exist in the life cycle. Gametes are released from one haploid adult and fuse with gametes similarly released from another to form a zygote (Figure 9.8). In most species of Ulva, the male and
female gametes are morphologically identical (isogamy). The zygote germinates to form a diploid plant, indistinguishable from the plant that produced the gametes, except for its complement of chromosomes. When the diploid plant is mature, it produces haploid zoospores by meiosis, which settle on an appropriate substratum and develop into haploid Ulva plants. This form of alternation of generations is called isomorphic, because both haploid and diploid forms look alike and each assumes an equal dominance in the life cycle. It is, however, more usual for alternation of generations to be heteromorphic, with the sporophyte and the gametophyte being physically dissimilar, and with one form or other dominating.