Plant Kingdom: Algae to Angiosperms Exam Notes

The plant kingdom holds everything from a single-celled alga in a pond to a hundred-metre redwood, and the examiner's interest in it is almost entirely in the order of that series. Plants are placed in five groups: algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Each step in the series adds something the group before it lacked, and the question papers test exactly those additions: a body divided into root, stem and leaf, conducting tissue, a seed, and finally a fruit around the seed.

How plants are classified

Carolus Linnaeus gave the first widely used system, based only on a few characters such as the stamens, and it is called artificial because it put unrelated plants together. Natural systems that followed used many characters, and modern systems add evidence from cytology, biochemistry and now molecular data. The classification students are examined on is Eichler's: the kingdom is divided into Cryptogamae, plants without seeds, which include the algae, bryophytes and pteridophytes, and Phanerogamae, plants with seeds, which include the gymnosperms and the angiosperms.

Algae

Algae are simple, chlorophyll-bearing, mostly aquatic plants with a thallus, that is a body not divided into root, stem and leaf. They are autotrophic and carry out about half of the photosynthesis of the earth. Three classes are asked.

Their uses are a standing favourite: agar, used in laboratory culture media and in ice cream, comes from the red algae Gelidium and Gracilaria; algin comes from brown algae and carrageenan from red algae; Chlorella is so rich in protein that it has been tried as food for astronauts; Porphyra and Laminaria are eaten in East Asia; and many algae are used as fertiliser.

Bryophytes

Bryophytes are called the amphibians of the plant kingdom because they live on land but still need water to complete fertilisation, the male gametes swimming to the egg. They have no vascular tissue, are attached by rhizoids instead of true roots, and the gametophyte, the haploid plant, is the dominant and independent phase. Liverworts such as Marchantia and Riccia and mosses such as Funaria, Polytrichum and Sphagnum are the standard examples. Sphagnum, the bog moss, holds water like a sponge and its partly decayed remains form peat, long used as fuel; mosses also form the first cover on bare rock and check soil erosion on slopes.

Pteridophytes

Pteridophytes are the first plants with vascular tissue, that is with xylem to carry water and phloem to carry food, and the first with a true root, stem and leaf. The sporophyte, the diploid plant, is dominant and bears spores in sporangia; the spore grows into a small independent gametophyte called the prothallus, which still needs moisture for fertilisation. Selaginella, Lycopodium, Equisetum the horsetail, Pteris, Adiantum the maidenhair fern and Marsilea are the names to remember. Selaginella and Salvinia produce two sizes of spore, a condition called heterospory, and this is regarded as the first step towards the seed habit.

Gymnosperms

In gymnosperms the ovules are not enclosed in an ovary, so the seeds lie naked on the surface of the cone scale and no fruit is formed. They are mostly woody perennials of cold and dry regions, pollinated by wind, with needle-like leaves and sunken stomata to cut water loss. Cycas, Pinus, Cedrus, Ginkgo, Araucaria, Ephedra and Sequoia are the usual examples. Three facts are asked often: Cycas bears coralloid roots that contain nitrogen-fixing cyanobacteria, the roots of Pinus carry a fungal partnership called mycorrhiza, and Ginkgo biloba is called a living fossil because it is the only surviving species of its order. Sequoia, the giant redwood, is the tallest plant known.

Angiosperms

Angiosperms are the flowering plants and the largest group in the kingdom. The ovules are enclosed in an ovary, which ripens into a fruit, so the seed is protected and can travel. Their life cycle has two features examiners like: double fertilisation, in which one male gamete fuses with the egg to form the zygote while the second fuses with the two polar nuclei, and triple fusion, which is that second event and which makes the endosperm triploid. Wolffia, a floating plant a few millimetres across, is the smallest angiosperm, and Rafflesia bears the largest flower. Angiosperms are divided into two classes whose differences are a certain question.

Alternation of generations

Every plant alternates between a haploid gametophyte and a diploid sporophyte. In most algae the haploid phase dominates and the pattern is called haplontic. In bryophytes and pteridophytes both phases are independent at some stage, which is the diplohaplontic pattern, with the gametophyte dominant in bryophytes and the sporophyte dominant in pteridophytes. In gymnosperms and angiosperms the sporophyte is the whole visible plant and the gametophyte is reduced to a few cells inside it, which is the diplontic pattern. The series read in this order explains the whole conquest of land by plants: first water for fertilisation, then conducting tissue, then the seed, then the fruit.