Short Notes:

Plants such as roses and cacti are protected from hungry animals with sharp, woody growths. On a rose, these growths are commonly called thorns, but they are really prickles that arise from the outer layer of the stem. On a cactus, the growths are called spines, and they are a form of leaf. Other plants with spines include holly, where the spines grow from the tips of the leaves, and stinging nettles, with spines that break and release a poison where touched. True thorns are modified stems, as in the honey locust and hawthorn. In some cases thorns sprout leaves.

Root, stem and leaf are the three basic parts of every plant, but in many cases these parts grow into structures that bear little resemblance to the original organ. This adaptation turns leaves into spines or tendrils. Different parts of a plant that arise from the same organ are called homologous features. The tendrils of a pea plant and the edible part of an onion are all homologous, since each is a modified leaf. In some cases, two plants may have features that look the same or have the same function, but that nevertheless developed from different organs. These are called analogous parts. An onion, potato, and sweet potato seem similar, but an onion is a modified leaf, a potato is a modified stem, and a sweet potato is a swollen root. in the same way, the tendrils of a pea and ivy are analogous, since the pea tendril is actually a leaf, and the ivy tendril is part of the stem.