Tropism and Movement in Plants Motion in two natural categories: 1. Tropism, (Greek, trope, meaning bending) which means that the direction of environmental stimulation determines the direction of movement. For example, stems that grow away from gravity or towards a light source. 2. Nastik, (Greek, nastos, meaning forced to come closer) which is triggered by external stimulation, but the direction of the stimulus does not determine the direction of movement. For example, the daily movement of leaves, or the opening and closing of stomata. Environmental changes that induce plant movement or other responses are called stimulation. The part of the plant that receives stimulation is calledreceptor.Once a stimulus is received it will be converted (transduced) into another form, often known as a signal, which is then transmitted into a motor response. That is what causes plant movement. Three stages of plant movement: 1. Acceptance 2. Transduction 3. Response Nasti Movement The movement of leaves or leaflets on compound leaves often shows nastic movement. The bending of the organ upwards is calledhyponasty, downward bending is calledepinasty. This leaf movement is caused by the presence of a pulvinus at the base of the petiole, leaf blade, or leaflet, but this movement also occurs in plants that do not have a pulvinus. Epinastic bending of leaves is caused by faster cell growth at the top of the petiole and leaf blade, compared to the bottom. The bending changes to hyponastic when the cells on the lower side change to grow faster. Niktinasti ; Nicotinastic movement is a rhythmic process controlled by the interaction between the environment and biological time. For example, the movement of leaves, from almost horizontal during the day to almost vertical at night. Hydronasty (or hygronasty) ; Hydronasty is the folding or rolling movement of leaves, but leaf rolling occurs as a response to water conditions, not to light. This movement occurs due to loss of turgor in thin-walled motor cells called bulliform cells. When turgor pressure decreases, cell turgidity remains on the underside of the leaf causing the leaf to fold. Tigmonasty; It is a nastic movement resulting from touch (Greek, thigma, meaning touch). This movement is seen in certain families such as mimosaceae, fabaceae. The clearest example is the daughter of shame (Mimosa sp .). The purpose of this response for plants is uncertain but one suggestion is that the folding of the leaflets will startle and repel insects before they can eat the leaves. The research results show that nasti movement is controlled by a new class of plant hormone called turgorin. Geotropism During growth, plant organs are influenced by various external factors. Among them can determine the direction of growth. Plants can react to those stimuli (for example light and gravity) by carrying out growth movements (groeth movement), reversible movements or trophic growth, namely the direction of growth depends on the arrival of stimuli. Geotropy, namely the direction of growth determined by gravitational stimuli. Plant organs can grow following positive, negative geotropy, diageotropy or plagiotropy. That the direction of growth is determined by gravity can be shown by experiments using a clinostat. The recipient of the geotrophic stimulus is thought to be starch grains found in certain cells at the root tip. The geotrophy mechanism is explained by the auxin theory, namely that auxin production in roots and stems is transported to the basalt. If the roots or stem are laid down, the auxin content is higher on the lower side so that growth will be stimulated (in the stem) or stunted (in the roots) and result in bending. The same information can be used to explain phototrophy events (the stimulus is light). Other trophic growths include hydrotrophy, aerotrophy, chemotrophy and others.