Stems are the plant’s scaffold, pipes, and renewable tissue factory
A vascular plant’s stem shares axis duty with the root: holding leaves and flowers to the light, shipping water and sugars in xylem and phloem, storing reserves, sometimes photosynthesizing, and growing new cells from meristems because typical plant cells live only one to three years.
Vascular plants are built around two main axes, the root and the stem. The stem holds up leaves, flowers and fruit, keeping leaves in the light they need for photosynthesis. It moves water and dissolved material between roots and shoots, stores nutrients for lean times, can photosynthesise itself where it has chloroplasts, and makes new tissue. Botanists also call it a culm, haulm, stalk or thyrsus, and a tree's woody stem is its trunk. Most stems grow above ground, though some run underground.
A stem is laid out in nodes and internodes. Nodes are small growth zones where one or more leaves attach, often with an axillary bud in the angle that can become a branch bearing leaves, cones or flowers. Nodes can also sprout brace roots or, on vines, tendrils. The internodes are the stretches between, able to lengthen from the base or the tip depending on species. Shoot is a looser word for fresh growth, meaning stems together with their leaves or flowers.
Inside run two pipelines. Xylem forms on the inner side and lifts water through transpiration pull, capillary action and root pressure. Phloem forms on the outer side from sieve tubes and companion cells, carrying food made in photosynthetic tissue to the rest of the plant. Between them lies the cambium, which divides to produce both. Since the ordinary cell survives just a few years, stem meristems keep producing replacement tissue annually.
Many stems are specialised. Onions, daffodils and tulips grow bulbs, short upright underground stems wrapped in fleshy storage leaves. Taro, crocus and gladiolus store food in corms, iris and most ferns spread by rhizomes, and garden strawberries send out runners that root at the nodes. Cactus pads are cladodes, flattened stems that do the work of leaves, and rose thorns are prickles grown from the stem's outer layers. A banana's trunk is a pseudostem of rolled leaf bases that can stand 2 to 3 metres tall.
Source: Plant stem
