Each plant organ originally evolved in the context of specific environmental imperatives related to terrestrial life. Roots anchor the plant and also absorb water and mineral nutrients. Leaves were adapted to optimize photosynthesis. Stems elevate the leaves, serve as a conduit from the roots to the leaves, and also generate new growth. However, each linage of plants have followed their own unique evolutionary path through time, and in many plant groups stems roots and leaves have become secondarily modified by natural selection in unusual and surprising ways.
These are examples of shoots (including both the stem and leaves) that have become secondarily modified.
Bulbs: These shoots are an adaptation for interrupted life....for existence in environments with a dormant season due to annual rainfall variations or to life in the temperate climates.
Thorns: The thorns of hawthorne are modified dwarf shoots.
Tendrils: Tendrils of grape are modified flowering shoots. In this link you can see an ontolical mistake where a shoot is one part tendril and one part flower shoot.
Rhizomes: While typically competition results in vertical stems, there are environments where horizontal stems are adaptive. These are termed rhizomes.
Tubers: Like bulbs these are adaptations for interrupted life. Irish potatoes are tubers. It is clear that they are stems when you realize that the eyes are axillary buds.
Corms: Like tubers and bulbs these are adaptations for intrrupted life. Often mistakenly called, bulbs, a corm is a solid underground stem, vertically oriented where tupically only one bud germinates at the end of dormancy.
Stolons: These are like rhizomes inthat they are oriented horizontally. However, they are adapted to generate new plants that are clones of the original.
Succulent/photosynthetic Stems: In extremely dry environments, often the leaf becomes reduced and non-photosynthetic, and the stem becomes the primary photosynthetic organ of the plant. A barrel shape provides a low surface to volume ratio which reduces water loss.
Cladophylls: If a plant becomes genetically boxed in, like the plants in the preceeding link, and the environment becomes more mesic, the same set of factors that caused the generic leaf to become thin and flat will work to make barrel-shaped, succulent stems, thin and flat too.
Succulence: In a dry environment, leaves may either become reduced, like in cacti, or, else, become succulent. Succulent leaves not only retain lots of moisture but also have a lower surface to volume ratio.
Stipular Spines: Stipules can become modified into spiines for defense.
Stipular Spines Modified for Mutualistic Ants: In bullhorn acacia, evolution has proceeded one more step. In this plant the stipular spines are hollow and house mutualistic ants. The leaves are further modified to provide yellow structures on the margins of the leaves which only function to feed the ants. The perioles bear glands that excrete nectar for the ants too.
Spines for Defence: The non-photosynthetic leaves of cacti have become modified for defence.
Tendrils: In peas the terminal leaflet of the leaf has become modified into a tendril for climbing.
Leaves modified for Reproduction: Typically, leaves are determinately growthed. The grow, function and then die without sustaining new growth. But Kalanchoe has leaves that have become modified to generate new plants.
Leaves Modified to Trap Animals a Great Source of Nitrogen:
Bladderwort: This flowering plant grows below the water and is greatly reduced. It is often mistaken for green algae. Its leaves entrap small animals such as Daphnia and even larval fish.
Pitcher Plants: These are adapted for life in marshy/boggy nitrogen poor soil. The leaves are passive but lethal to insects.
Venus Fly Traps and Sundew: These are in the same family. They are adapted for life in wet nitrogen poor soils and both entrap insects.
Movie of Venus Fly Trap: While both sundew and the venus fly trap respond actively to entrapped insects, the fly trap does so spectacularly!
Succulent Petioles of Celery: Stalks of celery are actually petioles. These crist succulent petioles were derived from selective breeding.
Storage: Like bulbs and tubers, this is an adaptation for interrupted life. Further, unlike stems and shoots, the primitive habit of roots are subterranean, where it is best to secure energy and propagative resources.
Climbing: Some vines will form adventitious roots that assist th eplant in clinging to a substrate. Poison ivy has this habit.
Crops Derived from Brassica oleracea:
Kale, collards,cabbage, kohlrabbi, brussels sprouts, cauliflower and broccoli were all derived fromthe same species of plant through selective breeding.