Question

Write two examples of plants with thorns.

Answer 1

Hint: Plants develop thorns and spines to prevent animal grazing. These thorns modified the structure of leaves. Thorns reduce transpiration in plants.

Complete answer:
Stem thorns are stiff sharp short structures present on the stem. These are modified stems and arise from buds. Thorns serve two purposes, the rescue transpiration and check on browsing by animals. They are hence classified as a mechanical defense of the plant. Thorns may or may not be branched since they are part of the stem. They are long pointed structures which may or may not be curved.
Thorns are present on axillary stems of a plant, examples of plants that have thorns are Citrus, Duranta, Bougainvillaea.

Additional Information: -Plants also have other thornlike structures called spines that have a similar function. Spines are modified leaves. They contain vascular tissue.
-Spines are further divided based on the structure modified into:
Petiolar spines: arise from petiole
Leaflet spines: arise from the leaflet
Stipular spines: arise from stipule
-Xerophytic plants like cactus are well known for their spines. These serve a dual purpose of reducing transpiration and protection from animal grazing.
-Another such structure is called a prickle, unlike spines and thorns, these are short and are an extension of the cortex and epidermis of the stem.
-Plants may also have spinose teeth which are in the form of sharp-pointed edges present on the margins of leaves, their main function is mechanical protection from grazing.

Note:
 -It is believed that thorns first evolved in xerophytic plants as a defense mechanism.
-Thorns regenerate fast and also provide protection from desert-dwelling animals.
-Humans also use this property of thorny plants, these plants are strategically placed at windows and around the perimeter of the property, along with private property, they are also used to protect crops in certain areas from dwelling animals.