why only rose and etc plants having thorn why do not all plants have thorn
Answers
Answer:
In most situations thorns on plant’s stem are modified leaves in terms of their ontogeny and histology. They are mostly seen in plants that grow in hot tropic climates (Xerophtes or Desert plants). The necessity for modifying leaves into thorns in these plants help in reducing water loss through transpiration due to reduced surface area. Mesophytes need not have to conserve water and hence they are devoid of thorns.
Since plants are typically anchored in place by their root system, they are not able to run away from predators. Therefore, plants have developed various defense mechanisms against common predators that eat them. Plants deploy a number of chemical and physical strategies for their protection from predators. Formation of spines, thorns instead of leaves or stem is one of the physical strategy adopted for this purpose. Anatomically, modified leaves are referred to as spines, modified branches are thorns and modified epidermal cells are caleed prickles. In most cases, they all are referred to thorns or spines.
A good example to appreciate that thorns are modified leaves can be seen when we plant a stem cutting of rose for inducing roots to get a rooted sapling. If you closely watch then you can see that new sprouts (new shoots/branch) always comes out from the axil of the thorn like all other normal plants without thorns.