In this article, we will discuss about the various defensive organs and defensive devices of plants.
(A) The defensive organs are: (1) Thorns, Spines and Prickles (2) Stinging Hairs (3) Sticky Glandular Hairs (4) A Dense Covering of Hair and (5) Cork and Bark. (B) The defensive devices are: (1) Poisonous or Repulsive Substances (2) Underground Modified Stems (3) Mimicry and (4) Myrmecophily.
The animal world is dependent on the plant world for the supply of food. A large number of animals are of the grazing habit and, naturally, such grazing means death and destruction for the plants. Plants are not capable of actively protecting themselves from the marauding animals. But, certain devices directly or indirectly help them in this struggle.
Plants do not adapt these devices out of their own will but such devices are developed in plants in course of evolution as any plant thus protected will have a point in its favour in the struggle for existence .
Most conspicuous of such devices are the armatures which act as stationary weapons or shields to ward off the marauders. Besides the armatures there are also some other devices which help in this effort on the part of Nature to protect her weaker creation—the plants.
A. Armature or Defensive Organs:
1. Thorns, Spines and Prickles:
Although these are of different origin, they are similar and serve the same general purpose of warding off the marauders. Thorns are deep seated stem structures while spines are modifications of different organs. Prickles are very superficial structures which grow only from the epidermis. Such are the prickles on rose which are uprooted with the slightest pressure.
Such superficial prickles are also common on the leaves and fruits of many plants like brinjal (Solarium melongena), Solarium suratiense, etc. These thorny structures are often found in greater abundance on the lower (i.e., more mature) parts of the plant.
Naturally, grazing animals do not like to disturb plants like the various Cacti , Argemone mexicana or umaranthus spinosus as plants like these present a uniform mass of spinous bodies which are detested and even dreaded by the former.
2. Stinging Hairs:
Certain plants have epidermal hairs, the tips of which are siliceous (therefore, glassy) and unicellular, containing a poisonous sap . On contact with animal skin these needle-like hairs break and pierce the skin injecting the poisonous substances. This causes irritation which may sometimes be very serious. These hairs are, therefore, called stinging hairs. Stinging hairs are commonly found in the nettles.
There are various types of nettles belonging to the genera Urlica, Fleurya, Laportea, Girardinia, etc., of Urticaceae and Tragia of Euphorbiaceae. They are herbs and shrubs of various heights and of different degrees of poisonousness.
Similar stinging hairs are found on the fruits of the extensive climber Mucuna pruriens (Papilionaceae). All these are naturally avoided by grazing animals.
3. Sticky Glandular Hairs:
These are also epidermal outgrowths like the stinging hairs. They occur on leaves, stems, fruits, etc. These glandular hairs secrete some sticky substance so that the plant parts stick to the face of the animal causing much dislike on the part of the latter.
They are, therefore, avoided by animals. Common examples are fatropha, Plumbago zeylanica, Boerhaavia, tobacco, Siegesbeckia, etc.
4. A Dense Covering of Hair:
A dense covering of hair on plants like Calotropis is always disliked by grazing animals so that they are saved from them.
5. Cork and Bark:
Cork and bark act like a shield so that the maturer parts of the plants arc avoided by the marauders.
B. Other Defensive Devices:
1. Poisonous or Repulsive Substances:
Many plants contain more or less poisonous substances so that they are avoided by animals either by instinct or by experience. These substances may be present in various forms:
(a) Latex:
The laticiferous cells or vessels may contain some fluid which is repulsive to taste or is poisonous and is much disliked by animals as in Calotropis, Opuntia, Thevetia, Nerium, different species of Ficus and Euphorbia, etc.
(b) Tannin:
Tannin, present in barks and as cell-content, is disliked by most insects.
(c) Alkaloids:
Alkaloids are deadly poisons found in many plants, e.g., nicotine in tobacco, morphine in opium poppy, quinine in cinchona, strychnine in nux-vomica, daturine in datura, caffeine in coffee, theine in tea, emetine in ipecac, atropine in belladonna, etc. Although strong poisons, they are important medicines in mild doses.
(d) Bitter taste and Offensive Smell:
Bitter taste and offensive smell also are caused by chemical contents and ward off many animals. The bitter taste of margosa, Momordica and the smell of Poederia, Blumea lacera and many Labiatae are useful in this sense. The inflorescence of Amorphophallus campanulatus also emits a fetid smell.
(e) Irritating Substances:
The raphides and the sphaeraphides in various aroids are well-known sources of irritation and are effective in saving the plants.
(f) Silica:
Silica on the epidermal cell walls of many grass leaves render them dangerously .sharp for grazing animals and are invariably avoided.
2. Underground Modified Stems:
Underground modified stems and some plants with fleshy roots have a peculiar memoirs where a big part of their lives is spent under the soil out of the reach of grazing animals. They are, thus, rendered safe for this period of their life.
3. Mimicry:
Certain plants strikingly resemble some dreaded animals so that they are carefully- avoided by other animals. This is known-as mimicry or copying other animals.
Many aroids resemble serpents and the most striking of them is Arisaema speciosa growing abundantly on the forest floors in Shillong and Daijeeling.
The plant body is often spotted and the spathe of the inflorescence (green or greenish-purple), which is variously striped or spotted, is shaped just like the hood of a cobra. The plants look menacing from a distance and are carefully avoided by the marauders.
4. Myrmecophily:
Another very interesting device is what is known as myrmecophily (literally, love of ants). These plants have something which is attractive to some war-like ants which then take their abode on such a plant and act as a troop of bodyguards against any marauding animal.
This is generally the case in fruit trees like mango, litchi, Syzygium jambos, etc. In certain aroids with corms it is found that the underground corm is divided up into compartments which then serve as a castle for dangerous ants so that the corm cannot even be touched.
The device is brought to its perfection in certain South American Acacias. In Acacia sphaerocephala the spiny stipules arc hollow and serve as shelters for the ants.
At the same time, certain food substances are secreted and accumulate as particles on the tips of leaflets. These are called Belt’s corpuscles. So, the plant keeps ready both food and shelter for the troop of bodyguards which are always ready to accept them in lieu of the defense service undertaken.