Which of the following statements about plant defences against herbiv…

The correct answer is Option (3) → Cactus produces highly poisonous cardiac glycosides, which is why cattle and goats are not seen browsing on it.

Plants cannot run away from herbivores, so they have evolved morphological and chemical defences. Thorns are the commonest physical defence. Many plants store chemicals that make the herbivore sick or stop it from feeding. The question asks for the one statement that does not match the book. The plant the book names for poisonous cardiac glycosides is Calotropis, a weed of abandoned fields, not cactus.

Option (1) Thorns, as in Acacia and cactus, are the most common morphological means of defence. — A correct statement. Thorns, as in Acacia and cactus, are the most common morphological defence.

Option (2) Many plants produce and store chemicals that make the herbivore sick when eaten or inhibit its feeding or digestion. — A correct statement. These are the chemical defences: the stored chemicals may make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or even kill it.

Option (3) Cactus produces highly poisonous cardiac glycosides, which is why cattle and goats are not seen browsing on it. — The incorrect statement. Cactus defends itself with thorns. The plant that makes highly poisonous cardiac glycosides, and is therefore not browsed by cattle or goats, is Calotropis.

Option (4) Nicotine, caffeine and quinine are produced by plants as defences against grazers and browsers. — A correct statement. Nicotine, caffeine and quinine, which we extract from plants commercially, are made by the plants as defences against grazers and browsers.

NCERT Extract:
Population Interactions – “… Plants therefore have evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns (Acacia, Cactus) are the most common morphological means of defence. Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. You must have seen the weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant. A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.”