Stem modifications

 

Modifications are changes in form and function to suit varied needs like storage of food, reproductive growth and survival through unfavourable seasons, vegetative propagation, mechanical support, protection, photosynthesis, etc. Modifications occur in underground, subaerial and aerial stems. Underground Modifications The stems of some plants may remain underground permanently, generally in a dormant state, and produce aerial shoots annually under favourable conditions. They contain sufficient amount of reserve food materials and bear roots and buds. Such stems can be used as ‘seeds’ to produce new plant. 

Rhizome- It is thick, prostrate and branched stem growing horizontally beneath the soil surface. Nodes are marked as dry scars. They bear scale leaves, and branches with buds in their axils. The lower surface of the nodes gives out small slender adventitious roots, e.g. Zingiber officinale (ginger), Curcuma domestica (turmeric), some ferns and many aroids. 

Tuber-  It is the swollen tip of the underground branch. It stores large amount of reserve materials primarily starch, e.g. Solanum tuberosum (potato). The eyes of potato are nodes at each of which 1-3 buds are produced in the axils of small scale like leaves. Cutting of tubers propagates potatoes. Each piece of tuber should contain an eye or node for vegetative propagation. 

Bulb- It is highly reduced stem represented by a small disc like structure upon which numerous fleshy scaly leaves are borne (which store food material). The disc and leaves together are called bulb. On the upper side, disc bears terminal bud surrounded by number of leaves. The axillary buds are present between the axes of leaves. The adventitious roots are borne on the lower side of the disc. Bulbs are of two types: Tunicated. In tunicated bulb, the scaly leaves overlap one another to form concentric circles, e.g. Allium cepa (onion). Scaly or imbricate. The scaly bulb is one in which fleshy scale leaves may lie loose without forming concentric circles on the discoid stem, e.g. Allium sativum (garlic). 

Corm- This is a small underground stem, like the rhizome, but the main axis grows vertical by an apical bud. It bears scale leaves at the nodes in whose axils axillary buds are formed. They form daughter corms. Adventitious buds arise usually from the base of the corm. The corm is much contracted and is principally a storage organ and is much swollen due to the storage of food materials, e.g. Amorphophallus (zamikanda), Crocus sativus (saffron), and Colchicum autumnale. 

Subaerial Modifications In some species, the stem is partly aerial and partly underground. 

Runner- These are produced by creeping herbs. The runners are slender stems that run or creep, over the ground, often for considerable distances. At every node, it is rooted and bears leaves above ground. Axillary buds form new aerial shoots. Many grasses, Oxalis and mint propagate by runners. 

Stolon-  It is a slender lateral branch that arises from the base of the main axis. After growing aerially for sometime the branch arches downwards to touch the ground, where its terminal bud gives rise to a new shoot and roots, e.g. Jasminum (jasmine), Fragaria (wild strawberry). 

 Sucker- It is like a runner but originates from the basal and underground portion of the main stem. It is shorter and stouter than a runner. It grows horizontally for a distance partly below ground, striking roots at the nodes and then emerges obliquely bearing a leafy shoot, e.g. Chrysanthemum. 

Offset- It is stout and short runner like branch, which bends at the tip and gives rise to a rosette of leaves above and roots below, e.g. Pistia, Eichhornia (water hyacinth). 

Aerial Modifications Such modified stems perform unusual functions and are aerial in position. They greatly vary in form but can be distinguished easily by their position in the axil of a leaf or at the apex. 

Stem-tendril- It is a thin, soft, long, wiry, leafless and spirally coiled structure, mostly found in climbers and helps the plant in climbing. When the tendril comes in contact with a support it coils round that holding firmly, and climbs up easily, e.g. Vitis vinifera (grape) – terminal bud modified into tendril. In Passiflora (passion flower) axillary bud is modified into tendril.

 Thorn-  A thorn represents an axillary branch of limited growth. It is hard, often straight, pointed and may be branched. Thorns serve as defensive organs. Examples are seen in Citrus medica (lemon), Aegle marmelos (wood apple), Duranta and Bougainvillea. 

Phylloclade-  It is found in most of the xerophytic plants. In such plants, stem or its branches become modified into flat, fleshy and green leaf like structure with distinct nodes and internodes. The leaves are modified either into spines or scales to reduce transpiration. Phylloclades serve as photosynthetic and storage organs, e.g. Opuntia (prickly pear), Muehlenbeckia (cocoloba), Euphorbia, etc. 

Cladode- A cladode is a phylloclade with one or two internodes only. It resembles a leaf. A cladode arises in the axil of a much reduced scaly leaf, e.g. Asparagus, Ruscus aculeatus.

 Bulbil- It is a special type of multicellular structure, and functions as an organ of vegetative reproduction. It is modified from either a vegetative bud or flower bud. In Dioscorea (wild yam), the bulbil occurs as a single fleshy axillary bud but in case of Oxalis (Indian sorrel), a large number of small bulbils occur on the tip of the tuberous root.