Which one of the following has the characteristics of both an animal as well as a plant?

Correct — D, (d) Euglena. Euglena is the standard textbook example of an organism that will not sit on either side of the plant-animal line, and it is the reason the two-kingdom classification of Linnaeus had to be abandoned.

Its plant-like features come from its chloroplasts. Euglena photosynthesises in the presence of sunlight, and the textbook notes that the pigments of the euglenoids are identical to those present in higher plants. On that evidence alone a botanist would claim it.

Its animal-like features are just as clear. It has no cell wall at all: in place of one it has a protein-rich layer called the pellicle, which is flexible and lets the organism change shape. It swims, using two flagella, one short and one long. It has an eyespot, or stigma, which is sensitive to light and steers it towards a bright region. And when it is deprived of sunlight it stops behaving like a plant altogether — the textbook says that euglenoids then behave as heterotrophs, preying on other smaller organisms. A cell that hunts is not a plant by any ordinary definition.

So Euglena photosynthesises like a plant and moves and feeds like an animal, which is exactly what the stem asks for. This is why Whittaker’s five kingdom classification places the euglenoids in Kingdom Protista, a kingdom created precisely to hold the unicellular eukaryotes that the older scheme could not accommodate: organisms that are cellularly eukaryotic, mostly single-celled, and mixed in their mode of nutrition. Most euglenoids are freshwater organisms found in stagnant water.

Why the others are wrong

• (a)Fern — A fern is unambiguously a plant. It belongs to the pteridophytes, the first group of plants to have proper vascular tissue — xylem and phloem — along with a true root, stem and leaves; it is photosynthetic, has cellulose cell walls, and reproduces by spores borne in sori on the underside of the fronds. Nothing about it is animal-like. It appears among the options because a candidate who is scanning for something unfamiliar or primitive may take a spore-bearing plant with no flowers to be a borderline case. Absence of flowers is not the same as absence of plant character.
• (b)Moss — A moss is also unambiguously a plant. Mosses are bryophytes, often called the amphibians of the plant kingdom because they live on land but need water for their sperm to reach the egg; they are photosynthetic, have no true vascular tissue and no true roots, and anchor themselves with rhizoids. The nickname is worth noticing precisely because it is the trap: ’amphibians of the plant kingdom’ is a metaphor about dependence on water for reproduction, not a claim that mosses have animal characteristics. A candidate half-remembering the phrase may take it for an answer to this question, which is very probably why the option is on the page.
• (c)Earthworm — An earthworm is unambiguously an animal — a segmented worm of the phylum Annelida, with a true coelom, a closed circulatory system, a nervous system and a complete digestive tract running from mouth to anus. It has no chloroplasts and cannot photosynthesise, so it has no plant characteristics whatever. Its reputation as the farmer’s friend, burrowing through soil and enriching it, associates it with plants in a candidate’s mind without giving it a single plant feature. Living among plants is not the same as sharing their characteristics, and this option is a good example of an association being offered in place of a fact.

Linnaeus divided living things into two kingdoms, Plantae and Animalia, and the scheme broke down under organisms it could not place. It made no distinction between prokaryotes and eukaryotes, between unicellular and multicellular organisms, or between photosynthetic and non-photosynthetic ones; bacteria, fungi and the single-celled eukaryotes all sat awkwardly inside it. R. H. Whittaker’s five kingdom classification answered this with kingdoms Monera, Protista, Fungi, Plantae and Animalia, using cell structure, body organisation, mode of nutrition, mode of reproduction and phylogenetic relationships as its criteria. Protista is the kingdom that absorbed the difficult cases: unicellular eukaryotes, with every mode of nutrition represented, including chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans. Euglena is the euglenoid the textbooks name. Its features divide neatly. Plant-like: chloroplasts, photosynthesis, pigments identical to those of higher plants. Animal-like: no cell wall but a flexible protein-rich pellicle, two flagella of unequal length, an eyespot for detecting light, and heterotrophic feeding on smaller organisms when kept in the dark. An organism that can feed either way is called a mixotroph, and mixotrophy is exactly what makes the plant-animal distinction useless at this level of organisation.

Key facts

• Euglena has both plant and animal characteristics and is the standard example of the failure of the two kingdom classification.
• Plant-like: it has chloroplasts and photosynthesises, and the pigments of euglenoids are identical to those of higher plants.
• Animal-like: it has no cell wall, but a protein-rich flexible layer called the pellicle.
• Animal-like: it has two flagella, one short and one long, and an eyespot sensitive to light.
• Animal-like: deprived of sunlight, euglenoids behave as heterotrophs and prey on smaller organisms.
• Euglenoids are placed in Kingdom Protista in Whittaker’s five kingdom classification, and are mostly freshwater organisms of stagnant water.
• Whittaker’s five kingdoms are Monera, Protista, Fungi, Plantae and Animalia, based on cell structure, body organisation, mode of nutrition, mode of reproduction and phylogenetic relationships.
• An organism that can feed both autotrophically and heterotrophically is called a mixotroph.
• Ferns are pteridophytes with true vascular tissue; mosses are bryophytes without it, called the amphibians of the plant kingdom because they need water for fertilisation; earthworms are annelids.

Common traps

• Taking ’amphibians of the plant kingdom’, the nickname of the bryophytes, as evidence that mosses have animal characteristics.
• Assuming a plant without flowers must be an intermediate form. Ferns and mosses are plants without qualification.
• Confusing an ecological association with a shared characteristic; the earthworm lives among plants and shares nothing with them.
• Placing Euglena in the plant kingdom because it photosynthesises. Its lack of a cell wall, its flagella, its eyespot and its heterotrophy in the dark all point the other way.

• practice — not a real PYQ

  In place of a cell wall, Euglena possesses

  • (a)a cellulose sheath
  • (b)a protein-rich pellicle
  • (c)a chitinous exoskeleton
  • (d)a calcareous shell

  Answer(b) a protein-rich pellicle

• practice — not a real PYQ

  In Whittaker’s five kingdom classification, Euglena is placed in

  • (a)Monera
  • (b)Protista
  • (c)Fungi
  • (d)Plantae

  Answer(b) Protista