For the most part, the trees we think of are all dicots, one of the few exceptions being palm trees. The few monocot trees remain short for several years before producing a stem strong enough to serve as a trunk, because, unlike dicots, their stems do not increase greatly in width once they begin to grow taller. The reason for this is that monocots vascular bundles usually lack a vascular cambium, so no new xylem or phloem cells can be produced. Because xylem is responsible for increasing thickness of stems, few monocots grow more than a few meters tall. It may be interesting to note that it is this production of xylem cells that causes tree rings.
Hormones are another thing that affect the growth of plants. Hormones are substances that are produced in one part of an organism that affect another part of the organism. Plant hormones are produced mainly in the apical meristems, young leaves, growing seeds, and developing fruits. Plant hormones control the branching out of the plant, the rate of growth, and how it responds to conditions in the environment.
The effects of hormones can vary a great deal based on the concentration of the hormone, and how it interacts with other hormones in the plant. Low concentrations of a hormone can cause the opposite reaction that high concentrations of the same hormone. Also, the same concentration of a hormone can have different effects on different plant organs.
Auxin is one such hormone. High concentrations of auxin induce the elongation of stem cells, but inhibit the elongation of root cells. Because of this, it causes stems to grow towards light, and against the pull of gravity, and causes roots to grow away from light and with the pull of gravity. In both roots and stems, auxin will concentrate on the side away from the light.
If a plant is knocked on its side, auxin will accumulate on the lower side of the stem, and cause it to bend upward. Because auxin also causes plants to grow against the pull of gravity, the plant will curve upward even in the absence of light.
Cytokinins are hormones produced in the roots which move upward in the plant. Several of its effects are the opposite of auxin’s. Where auxin stimulates cell elongation, cytokinins inhibit cell elongation and cause cells to grow thicker. Auxins inhibit growth of lateral buds, but cytokinins stimulate the growth of lateral buds. As you can see, the relative concentrations of auxin, cytokinins, and other hormones will determine how the plant will grow.
The responses to environmental stimuli that these hormones cause are called tropisms. A response to light is phototropism, a response to gravity is gravitropism, and a response to touch is thigmotropism. If a plant grows towards a stimulus, it has positive tropism, and if it grows away from a stimulus, it has negative tropism.