Modification of Stem

Stem modifications are grouped as aerial, sub-aerial or underground, depending on where the modified stem sits relative to the ground.

Aerial modifications keep the stem above ground but change its form for photosynthesis, storage or protection. The phylloclade is a green, flattened or angled stem/branch of unlimited growth with a series of nodes and internodes; it is characteristic of xerophytes whose leaves fall early and reduce to spines or scales, so the phylloclade itself takes over photosynthesis (also called cladophyll) - flattened in Opuntia, Phyllocactus and Muehlenbeckia, cylindrical in Casuarina, Euphorbia tirucalli and Euphorbia antiquorum. The cladode is similar but represents only one or two internodes, and its stem nature shows in the buds, scales and flowers it bears - cylindrical in Asparagus, flattened in Ruscus. Thorns are woody, sharp-pointed modified stems formed from either the axillary bud (Citrus, Atalantia) or the terminal bud (Carissa), used mainly for protection.

Sub-aerial modifications occur in plants with weak stems whose branches lie horizontally on or near the ground, mainly to propagate the plant vegetatively. The creeper/runner is a slender, prostrate stem that creeps along the ground and roots at each node it touches (Cynodon dactylon, Oxalis, Centella). The stolon is a slender lateral branch that first arches obliquely above the ground, loops over, and bends back down to root where it touches soil, forming an independent plantlet (Mentha piperita, Fragaria indica). The sucker arises from an underground stem and grows obliquely upward to give rise to a separate plantlet (Chrysanthemum, Musa, Bambusa). The offset, common in rosette-leaved aquatic plants, is a short, thick, leafless lateral branch that grows horizontally from a lower leaf axil before producing a rosette of leaves and roots at its tip (Eichhornia, Pistia).

Underground modifications are perennating, storage rootstocks found in perennial and some biennial herbs; unlike true roots they possess nodes, internodes, scale leaves and a terminal bud, even though - like roots - they lack a root cap and root hairs. The bulb is a condensed, conical/convex stem wrapped in fleshy scale leaves: a tunicated bulb has concentric scale layers (simple, e.g. Allium cepa, or compound, e.g. Allium sativum), while a scaly bulb has narrow, only partly overlapping scales (Tulipa); a pseudobulb is a short, erect, aerial storage stem of some epiphytic orchids (Bulbophyllum). The corm is a succulent underground stem with an erect growing tip, surrounded by scale leaves and bearing nodes/internodes (Amorphophallus, Gladiolus, Colocasia, Crocus, Colchicum). The rhizome grows horizontally underground with several lateral growing tips, and its conspicuous nodes and internodes are covered by scale leaves (Zingiber officinale, Canna, Curcuma longa, Maranta arundinacea, Nymphaea, Nelumbo). The tuber is a succulent, spherical/globose underground stem with many embedded axillary buds ('eyes') (Solanum tuberosum, Helianthus tuberosus). …