leaf: growth leaf: external leaf: internal leaf: modified
Plants often modify their stems for special functions. These modifications can have unusual and even bizarre shapes, and can grow above as well as below the ground.
Modified stems that grow above ground:
Stolons or runners-are horizontally oriented stems that grow along the soil surface. Their function is vegetative production. Example: number of grasses have stolons. Spider also plant has many stolons, each with a plantlet forming at the end of the stolon. Strawberries are normally propagated asexually by two different methods: runners (stolons) and crown division.
Tendrils and twining shoots-coil around objects and help support the plant.
Searcher shoots-are stems with long internodes that move in circles through the air seeking a support.
Cladodes or cladophylls-are flat, leaflike stems modified for photosynthesis.
Succulent stems-of plants have a low surface to volume ration and store large amounts of water and are common in desert plants.
Thorns-are modified stems that protect plants from grazing animals. Spines are modified leaves. How can you tell the difference?
Short ( spurs) and long shoots-long shoots have long internodes, while short shoots are rosettes and have short internodes. Apple spurs One of the simplest modified stems is the fruiting spur of an apple tree. Once the plant reaches maturity it begins to develop short branches (spurs) that grow very slowly. Spur growth may be limited to a few millimeters per year versus several inches of vegetative growth on other branches.
Offshoots Applies to lateral branches on stems of monocots. Date palm, pineapple, banana, and orchids produce offshoots. Bamboo also propagate from offshoots, if left unchecked they will form whole forests of plants...
Modified stems that grow below ground
Examples include tulips, hyacinths
Bulbs-are rosette stems surrounded by fleshy leaves that store nutrients. A bulb consists of a stem from which modified fleshy leaves develop. The fleshy leaves are circular and hollow; new leaves develop within the older leaves. Leaves are attached to the 'stem' at nodes and spaces between nodes are called internodes. The presence of nodes and internodes means that the bulb is actually a modified stem. The papery covering is called a tunica.
Note the basal plate and the lack of roots. Tunicate bulbs produce a new (adventitious--not pre-formed) root system annually from the basal plate. Roots appear soon after planting the bulb.
Rhizomes-are underground stems that grow near the soil surface. They typically have short internodes and scale leaves, and produce roots along their lower surface. They store food for renewing growth of the shoot after periods of stress.
Example: iris, ginger, canna
The corm is a relatively solid modified stem that has a few fleshy leaves.. The node is the point where the leaf attaches to the stem. The presence of nodes and internodes makes the corm a modified stem. The papery tunic on the outside of a crocus corm is the dead petiole from last year's leaf. Corms also have a nonpersistent root system that lives only one season .....In addition to normal fibrous roots, corms and some nontunicate bulbs have special roots (contractile roots) .Because new corms develop on top of old, they are closer to the soil surface. Corms too close to the surface could suffer winter injury. Contractile roots are capable of becoming deeply anchored and contracting to pull the corm deeper into the soil. Thus corms can change their depth in the soil.
Example: crocus. gladiolas
Tubers-are swollen regions of stems that store food for subsequent growth. The potato is an example. It is a stem because it has many nodes called eyes with spaces between eyes known as internodes. Potato tubers develop at the end of swollen underground stem structures, rhizomes.
Eyes of potatoes are really axillary buds which contain several small buds at each site. These buds can expand to form shoots which grow on to make whole plants. For fun see the potato play site :http://winnie.acsu.buffalo.edu/potato/. Although the common potato is a stem, the sweet potato is a modified root!
Crown. Generally that part of a plant at the surface of the ground from which new shoots are produced.
see also the stolens/strawberries above for another illustration
Spider plants, African violets are propagated by pulling shoots off of the crown. Asparagus spears develop from a crown that grows underground