Diversity in Plant Nutrition

Insectivorous plants

Insectivorous plants, such as the pitcher plant and Venus flytrap, still photosynthesise using chlorophyll, but they grow in nitrogen-poor soil, such as peat swamps. They trap and digest insects using modified leaves, absorbing nitrogen compounds from the prey to supplement what the soil cannot supply.

Parasitic plants

A parasitic plant, such as the dodder (a thin, twining, leafless stem), obtains water, minerals and manufactured food directly from a host plant using special structures called haustoria that penetrate the host's tissue. Total parasites like the dodder lack chlorophyll entirely and depend completely on the host, harming it in the process.

Epiphytes

An epiphyte, such as certain orchids or the staghorn fern, grows on the surface of another plant purely for physical support and better access to light, without taking nutrients from the host. Epiphytes still photosynthesise normally and absorb water and minerals from rainwater, humidity and decaying debris that collects around their roots, so they are not parasites.

Comparing modes of nutrition

How these plants relate to ordinary green plants

Nearly all plants are autotrophs: they build their own food from carbon dioxide and water using light energy in photosynthesis. The plants in this topic are special because they add a second strategy on top of, or in place of, ordinary photosynthesis.

Working out which single need is being met, nitrogen, ready-made food, or light and support, is the key to classifying them.

An insectivorous plant is still an autotroph; it photosynthesises normally and only tops up its nitrogen supply from trapped insects. A total parasite has lost that ability and depends fully on its host.

An epiphyte photosynthesises like any green plant but changes only where it grows. Comparing each one against a normal green plant makes the difference clear.

Adaptations to a difficult habitat

Each of these feeding strategies is an adaptation to a habitat where the ordinary plant lifestyle is difficult. Insectivorous plants live in bogs and peat swamps where the waterlogged, acidic soil holds little usable nitrogen, so trapping animals is a way to obtain it.

Their leaves are modified into pitchers, snap-traps or sticky hairs that catch and digest small animals.

Parasitic plants have solved the problem of making food by taking it ready-made, which is why a total parasite can survive without leaves or chlorophyll. Epiphytes face competition for light on the dark forest floor, so they grow high on other plants to reach sunlight while keeping their own photosynthesis.

Seeing each mode as a solution to a shortage, of nitrogen, of ready-made food, or of light, makes them easier to remember.

Common mistakes to avoid

Worked exam-style question

Question. Three plants are studied. Plant X grows on the trunk of a tree and photosynthesises; plant Y is a leafless, thread-like stem wrapped around a host with structures pushed into it; plant Z grows in a nitrogen-poor bog and has leaves modified into traps.

(a) State the mode of nutrition of each plant. (b) Explain how plant Y obtains its food.

(c) Explain why plant Z traps insects even though it is green.

Model answer. (a) Plant X is an epiphyte, plant Y is a parasitic plant, and plant Z is an insectivorous plant. (b) Plant Y pushes haustoria into the host's tissue and draws water, minerals and ready-made food directly from the host, so it does not need to photosynthesise.

(c) Plant Z can photosynthesise for its carbohydrates, but the boggy soil is poor in nitrogen; it traps and digests insects to obtain nitrogen compounds for making proteins and nucleic acids.

Practice question

Try it. A gardener finds an orchid growing high on a mango tree. The orchid has green leaves, and its roots cling to the bark but do not enter the wood.

(a) State the orchid's mode of nutrition. (b) Explain, with two reasons, why it is not a parasite.

Exam tip

Key terms

• Photosynthesis, how green plants make food from carbon dioxide and water.
• Chlorophyll, the green pigment that absorbs light for photosynthesis.
• Producer, an organism that makes its own food, like a green plant.
• Biodiversity, the variety of organisms, including different feeding strategies.

Source:SRC-DSKP-EN

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