(a) What are the major characteristics of Anthocerotopsida? Describe the gametophytic and sporophytic generations of Anthoceros…

(a) Anthocerotopsida, and the gametophyte and sporophyte of Anthoceros (20 marks)

Major characteristics of Anthocerotopsida (hornworts) (8 marks):

• The gametophyte is a flat, dorsiventral, lobed thallus of parenchyma, with no differentiation into tissues, and no scales.
• Each cell has usually one large chloroplast with a pyrenoid, unlike other bryophytes.
• Rhizoids are smooth-walled and unicellular; the ventral surface has slime pores leading to mucilage cavities that house the cyanobacterium Nostoc.
• Antheridia and archegonia are embedded (endogenous) in the thallus.
• The sporophyte is a long, green, horn-like capsule that keeps growing from a basal meristem, with a foot but no seta.
• The capsule has a columella (in Anthoceros), stomata in the epidermis, and spores mixed with sterile pseudoelaters, and splits from the apex downward by two valves.
• The sporophyte is photosynthetic but depends on the gametophyte for water and minerals.

Gametophyte of Anthoceros (6 marks). The thallus is dark green, rosette-like and 1 to 2 cm across, lobed and fixed by rhizoids. The cells are parenchymatous, each with a chloroplast and pyrenoid. Slime pores on the ventral surface open into cavities with Nostoc colonies. Species are monoecious or dioecious. Antheridia develop in groups in chambers roofed by two layers of cells on the dorsal surface, and each produces biflagellate sperms. The archegonia are sunken in the thallus with only the neck projecting. Vegetative propagation is by fragmentation, tubers and gemmae, and fertilisation needs water.

Sporophyte of Anthoceros (6 marks). After fertilisation the zygote forms an embryo that develops into a foot embedded in the thallus and a slender, erect capsule 2 to 5 cm long or more, without a seta. A basal meristem adds tissue continuously, so spores ripen from the tip downward. In section the wall has an epidermis with stomata and a layer of chlorophyll-rich cells; inside are the central sterile columella and the sporogenous tissue, which forms spore tetrads and pseudoelaters. The mature capsule splits into two valves and the spores are shed and germinate into new gametophytes.

(b) Laminariales: salient features and economic importance (15 marks)

Salient features (10 marks):

• They are large brown algae (Phaeophyceae), the kelps, of cold marine waters, such as Laminaria (Saccharina), Macrocystis, Nereocystis, Ecklonia and Undaria; Macrocystis can exceed 40 m.
• The pigments are chlorophylls a and c and fucoxanthin; the food reserves are laminarin and mannitol; the wall contains cellulose and alginic acid.
• The sporophyte is large and differentiated into a root-like holdfast (hapteron), a stalk (stipe) and a flat blade (lamina), with some kelps also having gas bladders.
• Growth is from an intercalary meristem at the junction of stipe and blade; the stipe has an outer meristoderm, a cortex and a medulla with trumpet hyphae that transport food.
• Alternation of generations is heteromorphic: the large sporophyte bears sori of unilocular sporangia, which produce zoospores by meiosis, and these grow into microscopic, filamentous male and female gametophytes.
• Reproduction is oogamous, and the zygote grows directly into a new sporophyte.

Economic importance (5 marks):

• Alginates from the kelps are used as thickeners and stabilisers in ice cream, paints, textiles, paper, dental impressions, wound dressings and drug capsules.
• Kombu (Saccharina) and wakame (Undaria) are eaten as food; kelp is a source of iodine, mannitol and animal feed and is used as manure.
• Kelp forests support fisheries and coastal ecosystems.
• Stipes were used as laminaria tents to dilate the cervix.

(c) Comparison of the ovule-bearing structures of Cycas and Pinus with labelled diagrams (15 marks)

Cycas (5 marks). The ovules are borne on megasporophylls, which are leaf-like, brown, hairy structures arranged in a loose rosette at the stem apex, with no cone. Each has a stalk, a fertile middle region carrying 2 to 6 large ovules on its margins, and a sterile pinnate tip. The ovules are orthotropous and have one three-layered integument.

Pinus (5 marks). The female cone is compact, with an axis bearing spirally arranged compound scales. Each scale consists of a small sterile bract scale and, above it, a larger woody ovuliferous scale, a modified dwarf shoot, carrying two anatropous ovules on the upper surface at its base with the micropyles turned toward the cone axis. Each ovule has a single integument.

Comparison (5 marks):

• Organisation: a loose rosette of leaf-like megasporophylls in Cycas, against a compact cone of bract and ovuliferous scales in Pinus.
• Number of ovules: 2 to 6 or more per sporophyll, against 2 per scale.
• Position: marginal in Cycas, adaxial and basal in Pinus.
• Ovule orientation: orthotropous in Cycas, anatropous in Pinus.
• Seed: large, fleshy and orange-red in Cycas, winged in Pinus.
• Development: Cycas seeds mature in about a year, and Pinus cones take two to three years from pollination.