the stem
A meat used to hold a plant upright is a "steak." This term can refer to either a cut of meat or a wooden or metal rod used for support in gardening.
A plant cells consists of a cell wall (it hold the plant upright) and contains chlorophyll to feed itself.
They don't hold you upright, more like sideways...
Roots
A stem or stalk typically holds up the flower in most plants. This part of the plant provides support for the flower to remain upright and accessible for pollination. Some plants may also have specialized structures such as bracts or peduncles to hold up the flower.
Turgid walls help hold the plant upright by internal water pressure. A adaption that allowed plants to acquire the land as a niche.
Those are called the "eyes" of the potato and sprout when the potato isn't as fresh any longer; these are called seed potatoes. You can actually plant the potato "eyes" (or the whole potato) and they will grow more potatoes, but you have to do it at the right planting time for where you live. You can also sprout them in a glass of water indoors and they will grow long vines, much like a house plant. To do this, insert toothpicks around the center of the potato so they will hold the top half out of the water, otherwise the whole potato will rot.
Plant stems hold the plant upright an carry nutrients from one part of the plant to the other.
To act as a conduit for sap and to hold the flowers upright.
... is to hold testtubes in upright position.
A frame (commonly) of wood serving to hold a canvas upright, or nearly upright, for the painter's convenience or for exhibition.