Specialized Leaves and Stems
Although typical shoots are erect with photosynthetic leaves, over evolutionary time a great assortment of modifications of the basic body plan have arisen. Some clearly benefit storage of materials, others assist in vegetative reproduction (reproduction without seeds), various alterations deter herbivores, and many are simply innovations in ways to hold the shoot upright. The most bizarre of all may be the leaves of the insectivorous plants that are modified to ensnare and digest hapless insects and other small organisms. Some drown their victims in vase‐like rainwater‐filled petioles while others glue them to the leaf with sticky digestive enzymes. The Venus' flytrap, on the other hand, snaps its leaves together rapidly enough to enclose the unlucky insect that alights on the trigger hair.
You can see many of the modifications in common garden and edible plants. For example:
- Bulbs are underground buds with the stem reduced to a small knob on which fleshy storage leaves are clustered (e.g. dry onions).
- Tubers are fleshy underground stems modified to store starch (e.g. white, or Irish, potatoes). The “eyes” are the nodes with an axillary bud in each (the peel is periderm tissue). Sweet potatoes are roots.
- Rhizomes are horizontal underground stems with nodes, internodes, dry scale leaves, and adventitious roots (e.g. fresh ginger “roots” sold in grocery stores are rhizomes). Canna lilies, iris, and many grasses have rhizomes with which they are propagated.
- Corms are upright underground fleshy stems covered by leaves reduced to dry, covering scales (e.g. gladiolus and crocus). Note that corms store reserve food in stem tissue, and bulbs in leaf tissue.
- Thorns are woody, sharply pointed branch stems (e.g. honey locust).
- Spines are small, unbranched, sharp outgrowths of leaf tissue in which the parenchyma is replaced by sclerenchyma (e.g. cactus).
- Prickles are small pointed outgrowths from the epidermis or cortex of the stem (e.g. rose and raspberry).
- Cladophylls are flattened main stems that resemble leaves (e.g. butcher's‐broom, greenbrier, and some orchids). Edible asparagus shoots left to grow produce many small fern‐like cladophylls.
- Stipules are paired scales, glands, or leaf‐like structures at the base of the petiole formed from leaf or stem tissue (e.g. black locust).
- Bracts are modified leaves at the base of flowers or flower stalks. Some are highly‐colored and resemble petals (e.g. the red “petals” of poinsettia are bracts surrounding the small, yellow flowers).
- Tendrils can be exclusively leaf tissue (e.g. pea leaflets, nasturtium petioles, or cucumber leaves that twine and aid in supporting the shoots) or they can be modified special shoots with thin, modified stems (e.g. morning glories, grapes, and Boston ivy).
- Stolons, sometimes called runners, are thin, above‐ground, horizontal stems of indeterminate growth and long internodes that grow out from a parent plant and produce young plants at their tips (e.g. strawberry plants, and a host of the most pernicious garden weeds).