Read Stem Modifications by Structure and Job
A modified stem retains stem clues such as nodes, internodes and buds while taking on storage, climbing, defence or propagation roles.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Recognise stem forms
Runners (Oxalis), stolons and offsets spread near the soil surface; rhizomes grow underground and store food/perennate; suckers and offsets can form daughter shoots. Opuntia has a photosynthetic phylloclade; Citrus/Bougainvillea thorns are defensive stem branches.
Distinguish organs by origin
Tendrils may be modified stems or leaves depending on the plant: cucurbits have stem tendrils, pea has leaf tendrils. Onion scales are leaves, not stems. Several aquatic plants produce daughter rosettes at offset nodes.
Identify storage organs
Potato is a stem tuber, beet stores reserves in a root, and onion is a bulb formed from a compressed shoot axis and fleshy leaves. Match the named plant to the modified organ, not merely to where it grows.
Recognise a parasitic stem connection
Cuscuta is a stem parasite. Its peg-like haustoria penetrate a host stem and connect with the host vascular tissue to draw water, minerals and photosynthate. In a transverse section, an invasive bridge from parasite to host is a haustorium, not cambium or phloem.
OlympIQ exam lens
- ✓Oxalis: runner; ginger: rhizome; Opuntia: phylloclade.
- ✓Rhizomes and suckers can support vegetative propagation.
- ✓Pea tendril: leaf; cucurbit tendril: stem.
- ✓Cuscuta haustoria tap a host stem's vascular tissue.
A similar-looking tendril can have different organ origins in different species.
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