Plants without woody tissue employ cell structures and natural processes to maintain upright growth habits. Plants can support themselves at substantial heights using turgidity, the pressure exerted by water on certain internal parts of the plant. The ability of non-woody plants to stand upright thus depends on water, which they use for transpiration and a number of other essential functions.
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Plant Cell Structure
Plant cells consist of a nucleus, vacuole, mitochondria and other organelles surrounded by a rigid cell wall. Plant cells can be broadly divided into two basic types: vascular and nonvascular. Vascular plants have a cell structure that gives them the rigidity to sustain upright growth, whereas nonvascular plants are typically limited to heights of a few inches or less. The cell walls of vascular plants provide support, shape, pressure control and diffusion regulation. They also dictate the direction and rate of growth.
Turgor pressure is what allows plant cells to maintain upright growth. It is the pressure exerted on the cell wall of a plant by the internal contents of the cell, which is maintained by the vacuole. The vacuole fills with fluid or deflates as the surrounding membrane, called the tonoplast, pumps water in and out of its interior. This puts pressure on the walls of the cell, leading to cell elongation and providing growing plant material with the rigidity to stay upright.
Turgor pressure is maintained by water uptake resulting from transpiration. Plants transpire by evaporating water through the stomata in their leaves, which pulls water from the roots upward through the plant. Transpiration gives plant cells the water they need to maintain turgor pressure, similar to a balloon inflating. This turgidity allows vascular plants to grow to greater heights than their nonvascular counterparts, enabling them to make better use of available light. Transpiration is also responsible for plant cooling and the movement of minerals and sugars throughout the plant.
Wilting occurs when a plant is deprived of water and the vacuole inside the cell deflates, reducing pressure on the cell walls. As the plant loses water, the vacuoles are drained of fluid to support other functions vital to life. As turgor pressure drops, it becomes harder for plant cells to maintain their rigid structure, and the resulting flaccidity causes drooping stems, leaves and branches. Transpiration becomes more difficult as wilt increases, and plant death can result if water is not provided.