Exam practice

There are millions of kinds of living things, too many to study one by one. Classification sorts them into groups by their similarities and differences, so that by studying a few groups we learn about all of them. It makes it easy to identify a new organism (once we know its group, we know many of its features), gives every organism one agreed scientific name, and shows how organisms are related to each other and how they evolved.

Both have a flat thallus, which is why they look alike. But Fucus lives in the sea, has no rhizoids, and its fertilization doesn’t depend on a film of water on land, so it is an alga. Marchantia lives on land in moist, shady places, is fixed to the soil by rhizoids, makes its gametes in antheridia and archegonia, keeps its sporophyte growing on the gametophyte, and needs water for fertilization: these are the features of a bryophyte.

A fruit develops from the ovary of a flower. Gymnosperms have no flowers, only cones, and their cones have no ovary: the ovules lie open on the cone scales. So there is nothing to become a fruit, and their seeds stay naked.

A class is decided by many features together, not one. The crocodile has the main features of reptiles: its body is covered with dry, horny scales; it is cold-blooded; it breathes with lungs; fertilization is internal; and it lays shelled eggs on land. It has no hair, no milk and no feathers. Its four-chambered heart is an exception (shared with its closest relatives, the birds).

One flies and one swims, but both have the defining features of mammals: they have hair (a whale only a few bristles), they give birth to their young and feed them milk from mammary glands, they are warm-blooded, have a four-chambered heart and breathe with lungs. Their different shapes are adaptations to different ways of life; the basic features they share put them in class Mammalia.

Riccia is a bryophyte. It lives on land, but its male gametes must swim through water to reach the egg in the archegonium. Like an amphibian animal, which lives on land but must return to water to breed, it needs both land and water to complete its life cycle.

All three have a body divided into true root, stem and leaf, and all have vascular tissue (xylem and phloem). Those are the features of the division Tracheophyta. Their differences (no seeds, naked seeds, seeds in fruit) only decide their subdivisions: Pteridophyta, Gymnosperms and Angiosperms.

Three: Algae, Bryophyta and Tracheophyta. (Tracheophyta is further divided into three subdivisions: Pteridophyta, Gymnosperms and Angiosperms.)

• Their cells are eukaryotic, with a cell wall made of cellulose.
• They have chlorophyll and make their own food by photosynthesis: they are autotrophs.
• They store food as starch.
• Most are multicellular; a few, like Chlamydomonas, are unicellular.
• Most are fixed in one place and grow throughout life.
• They include non-flowering plants (algae, bryophytes, pteridophytes, gymnosperms) and flowering plants (angiosperms).

• Both are tracheophytes: they have vascular tissue (xylem and phloem).
• Both have a body divided into true root, stem and leaf.
• Both produce seeds (both are seed plants, spermatophytes), with an embryo, stored food and a seed coat.
• Both make pollen, so fertilization doesn’t need a film of water.
• In both, the sporophyte is the main plant; the gametophyte is tiny and hidden inside the cone or flower.
• Both are autotrophs, and many are large woody trees.

Partly, but not reliably. Leaves give useful clues:

• A large, feather-like leaf with brown sori underneath, or a young leaf coiled like a fiddlehead, is a pteridophyte.
• Needle-like or scale-like leaves usually mean a gymnosperm (pine, deodar, juniper).
• A broad leaf with net or parallel venation usually means an angiosperm.

But leaves can mislead. Cycas, a gymnosperm, has big feather-like leaves that look like a fern’s; a fern leaf without sori (a trophophyll) gives little away; some angiosperms have thin green needles that look just like pine needles (Casuarina and asparagus; though in these the needles are really thin green stems), while some gymnosperms have broad leaves (Ginkgo, Gnetum). The subdivisions are defined by seeds, cones, flowers and fruits, so to be sure we need to see those.

• They live in moist, shady places.
• Their body is divided into true root, stem and leaf, but they have no flowers and no seeds.
• They have vascular tissue (xylem and phloem).
• In ferns, the stem is usually an underground rhizome, and the leaves are large and feather-like.
• The underside of some leaves (sporophylls) has sori, with sporangia that make haploid spores.
• A spore grows into a small, heart-shaped gametophyte, the prothallus, which makes both male and female gametes; fertilization needs water.
• They show alternation of generations, with the sporophyte dominant.
• Examples: fern, Pteris, Lycopodium, Selaginella, Equisetum.

No. Flower parts in threes or sixes are a typical monocot feature, but not a certain one: some dicots also have flowers with parts in threes (the bay-leaf tree, tejpat, and other laurels, for example), and in some flowers the parts are hard to count. Roshni should check several features: parallel or net venation in the leaves, fibrous or tap roots, scattered or ringed vascular bundles in the stem, and, best of all, one or two cotyledons in the seed. If most features agree, the identification is reliable.

• i.Similarities: both are multicellular, autotrophic plants; both have true roots, stems and leaves; both have vascular tissue (both are tracheophytes); both make seeds. Differences: in the table below.
• ii.The orange is an angiosperm (class dicotyledon), because it bears flowers and its seeds are enclosed in a fruit. Cycas is a gymnosperm, because it bears cones instead of flowers and its seeds are naked.
• iii.The orange is more advanced. Its flowers attract insects that carry pollen exactly where it is needed; its ovules are protected inside an ovary; and its fruit protects the seeds and helps spread them (animals eat the fruit and drop the seeds far away). These features evolved later than cones and naked seeds.

Differences for (i):

D: a fish is a vertebrate (it has a backbone) and aquatic (it lives in water).

A sponge digests its food inside its cells (intracellular digestion). Water flowing through its canal system carries tiny bits of food and bacteria. Special cells lining the inside, the collar cells, beat their whips to keep the water flowing and catch these particles, taking each one into a food vacuole, where it is digested. Wandering cells (amoebocytes) then carry the digested food to other cells. Undigested waste leaves with the water through the osculum.

He should observe its external features carefully and ask questions in order, like a dichotomous key:

1. Does it have a backbone? (Vertebrates have a head, a skeleton inside, and usually four limbs.) If yes, it is a chordate, and he can find its class: scales and fins (Pisces), moist skin (Amphibia), dry scales (Reptilia), feathers (Aves), hair (Mammalia).
2. If not, does it have jointed legs and a hard outer skeleton? Then it is an arthropod (insect, spider, centipede).
3. Is it soft, with a muscular foot and maybe a shell? Mollusca (snail, slug).
4. Is it a worm? If its body is made of rings: Annelida (earthworm). If round, smooth and pointed at both ends: Nemathelminthes. If flat: Platyhelminthes.

On land he won’t find sponges, coelenterates or echinoderms, which live in water. The key above works exactly this way.

The whale, a mammal, is more developed than the shark, a fish:

• it has a four-chambered heart, which keeps oxygen-rich and oxygen-poor blood apart; the shark has two chambers;
• it is warm-blooded, so it stays active even in icy water; the shark is cold-blooded;
• it breathes air with lungs;
• it gives birth and feeds its calf milk, and cares for it for a long time, so more of its young survive;
• it has a larger, more complex brain.

• i.Both are in phylum Chordata (subphylum Vertebrata). The bat is in class Mammalia; the eagle in class Aves.
• ii.Similarities: both fly with forelimbs changed into wings; both are warm-blooded with a four-chambered heart and breathe with lungs. Differences: the bat has hair, the eagle feathers; the bat gives birth and feeds its young milk, the eagle lays eggs. (Also: the bat has teeth, the eagle a beak.)
• iii.Look inside the wings: both are built from the same bones, one upper-arm bone, two forearm bones, wrist bones and fingers, the same bones as in our arm and a whale’s flipper. That they share the same bones tells us both came from a common four-limbed ancestor. But the wings are made differently (skin between long fingers in the bat; feathers along the arm in the eagle), which tells us flight evolved separately in each line, after their ancestors had split: each changed the old limb in its own way.

Classification sorts living things into groups by their similarities and differences; evolution is the slow change of living things from simpler to more complex forms over many generations. The two are linked: organisms that share many features are put in the same group, and they share many features because they evolved from a common ancestor recently; groups that share few features split from their common ancestor long ago.
Classification also shows the order of evolution. Prokaryotes (Monera) came first, then single eukaryotic cells (Protista), then multicellular fungi, plants and animals. Among vertebrates, the heart goes from two chambers (fish) to three (amphibians, reptiles) to four (birds, mammals), showing advanced animals evolved from less developed ones. And because every living thing is made of cells, and every group shares some features with the others, all living things share a common ancestry.

See Figure 2.5. In words: Kingdom Plantae → three divisions: Algae, Bryophyta, Tracheophyta. Tracheophyta → three subdivisions: Pteridophyta, Gymnosperms, Angiosperms. Angiosperms → two classes: Monocotyledons and Dicotyledons. Write one or two examples under each group.

See Figure 2.19. In words: Kingdom Animalia → invertebrates, eight phyla: Porifera, Coelenterata, Platyhelminthes, Nemathelminthes, Annelida, Arthropoda, Mollusca, Echinodermata; and phylum Chordata → four subphyla: Hemichordata, Urochordata, Cephalochordata (together the Protochordata) and Vertebrata → five classes: Pisces, Amphibia, Reptilia, Aves, Mammalia. Write one or two examples under each group.

• i.The fish.
• ii.The tiger, a mammal.
• iii.Both are vertebrates; both are cold-blooded (poikilothermic); both have a three-chambered heart; both breathe with lungs as adults; both are oviparous (most snakes lay eggs).
• iv.The pigeon has air sacs connected to its lungs, and the fish has an air sac, its swim bladder.