Short answers, pulled from the story.
A plant stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers, and fruits, transports water and dissolved substances between the roots and shoots in the xylem and phloem, engages in photosynthesis, stores nutrients, and produces new living tissue.
Xylem arises from the cell facing inside and transports water through transpiration pull, capillary action, and root pressure. Phloem arises from the cell facing outside, is made of sieve tubes and companion cells, and distributes food from photosynthetic tissue to other tissues. The two are separated by the cambium, which divides to form xylem or phloem cells.
Nodes are the points of attachment for leaves and can hold one or more leaves, sometimes bearing axillary buds, adventitious roots, or tendrils. Internodes are the intervals that distance one node from another and can elongate from their base or their extremity depending on the species.
Dicot stems have pith in the center with vascular bundles forming a distinct ring, and many show secondary growth. Monocot stems scatter vascular bundles throughout, rarely produce secondary growth, and are seldom woody apart from palms and bamboo. All gymnosperms are woody, and most produce only tracheids in their xylem rather than vessels, with wood that often contains resin ducts.
Sapwood is the outer layer of secondary xylem that is still active in fluid transport and contains live parenchyma cells. Heartwood is the inner secondary xylem that no longer transports fluid and contains no living cells, serving only as structural support and appearing darker due to deposited and oxidized secondary metabolites such as polyphenolic compounds.
Plant stems supply staple crops such as potato and taro, sugar from sugarcane and maple trunks, and vegetables like asparagus, bamboo shoots, cactus pads, kohlrabi, and water chestnut. Stems also yield medicines including quinine, camphor, and curare, along with wood, paper, cork, rubber, tannins, and amber, with the earliest known paper made from papyrus stems by the ancient Egyptians.
Tree rings form from the seasonal variation in growth from the vascular cambium, which creates yearly rings in temperate climates. They are the basis of dendrochronology, which dates wooden objects and associated artifacts, and of dendroclimatology, the use of tree rings as a record of past climates.