What makes up a plant? Well, there are the flowers, the leaves, the buds (which are future leaves or stems in embryo), the stems and the roots. All of these are linked together by the vascular system, which moves water and minerals up from the roots to the leaves. Any water that remains after transpiration (evaporation via pores in the leaves) is then redistributed throughout the plant.
Trees and shrubs vary enormously in the size and shape of their above-ground growths. A stem or trunk may be long, short, thick or thin, depending on the species and the way it has been trained. The reason why a woody plant develops a branched habit is to expose the maximum leaf area to the sun, and therefore optimise ‘photosynthesis’. Through the trunk, water and nutrients are transported from the roots to the leaves. Conversely, foods taken in or manufactured by the leaves are returned through the trunk to build new root tissue, or stored as food.
It is worth remembering that new plants are being developed all the time, and often it is hardiness, and other weather tolerances, that are being bred into them. Essentially you need to know these two points:
Some plants dislike persistently damp conditions (alpines and bulbs, particularly), whilst others dislike persistently dry conditions (including moisture-loving plants such as bog garden plants, as well as fruiting plants and vegetables like runner beans and celery). Some plants dislike wind (fleshy leaved plants and climbers such as clematis, for example). And some plants grow best in the full glare (and heat) of the sun (such as pelargoniums and most summer bedding annuals).
So you can see that, to get the best from your garden, you need to know about your plants as well.