Final answer:
A stem without internodes used for photosynthesis like in Opuntia is called a cladode or a photosynthetic stem. These modified stems are adaptations in arid environments, allowing plants to perform photosynthesis with minimal water loss.
Step-by-step explanation:
A stem without internodes, which is used for photosynthesis like in the case of Opuntia, is referred to as a cladode or a photosynthetic stem. Cladodes are modified stems that take over the role of leaves in photosynthesis when the plant has reduced or no leaves at all, which is a common adaptation in arid environments. In the case of Opuntia, the green, flat, pad-like structures often assumed to be leaves are actually cladodes. They allow the plant to perform photosynthesis while conserving water, which is a valuable adaptation in deserts and other dry habitats.
In plants, internodal growth may be delayed or absent, as seen in plants with a rosette form, where multiple leaves are very close together. The presence or length of internodes can be influenced by environmental factors and may change with the stage of plant growth. For example, in some cases, a plant that has short internodes (rosettes) may 'bolt', rapidly elongating its stem during a particular phase of its lifecycle.
Stems are integral parts of vascular plants, containing nodes and internodes, where nodes are points of attachment for leaves, aerial roots, and flowers. Internodes are the regions between two nodes, and plants with rosette forms or like the Opuntia have minimized their internode lengths or don't have them at all to save water and adapt to their environment.