The movements which allow organs of plants to move in or the other direction is called movements of curvature. These movements are of two types i.e. mechanical (found in non living organs of plant) and another is vital movements (found in living organs of plants).
1.Mechanical movements (Hygroscopic movements) :
These movements depend upon the presence or absence of water, e.g., peristomial teeth of moss protrude out when the capsule is dry and curve inside the capsule when the capsule is wet. When water is lost from the annulus of the sporangia of fern, it bursts from stomium and spores are thus liberated out. Similarly the spores of the Equisetum coil and uncoil in the presence or absence of water respectively.
2.Vital Movements :
These movements are of two types.
(i) Autonomous movements :
These movements are of two types—
(A) Movements of variation—In Indian Telegraph plant (Desmodium gyrans) a member of Papilionatae, the leaf has a terminal leaflet and two lateral leaflets. During day time the lateral leaflets move up and down, due to variation in turgidity.
(B) Movements of growth—These movements are due to unequal growth in different parts of an organ. These are of the following types—
(a) Nutation—In some cases apex of a branch or an organ e.g., tendril becomes spiral due to more growth on the outer side.
(b) Hyponasty—These movements are usually found in leaves, petals etc., when abaxial or morphologically lower surface grows more than the upper surface, resulting in folding of leaves.
(c) Epinasty—When a leaf or petal shows more growth on the adaxial or morphologically upper surface the organ curves down wards. It is this type of curvature that occurs in unfolding of leaves.
(ii) Paratonic or Induced Movements :
These movements are of the following types—
A. Tropic Movements (Directive movements).
B. Nastic Movements (Nondirective movements).
A. Tropic Movements (Tropisms) :
These are movements of curvature brought about by more growth on one side and less growth on the opposite side of plant organ induced by some external stimuli. Depending upon the nature of stimuli these movements are of the following type—
(a) Phototropism (Heliotropism)—When a plant organ curves due to unilateral light stimulus it is called phototropism. Some parts of the plant e.g., stem moves towards light. These organs are called positively phototropic. Some other organs e.g., roots move away from light and they are called negatively phototropic. If we keep a plant in a dark chamber with an opening on one lateral side the stem tip moves towards light i.e., towards opening.
(b) Geotropism—Growth movements induced by the stimulus of gravity are known as geotropism. Growth curvature towards the gravitational force is known as positive geotropism (main root) and growth away from the gravitational force is known as negative geotropism, (stem). When an organ grows at right angles to the direction of gravity it is said to be diageotropic (rhizome, stolon). When an organ moves (curves) 0 to 90 or 90 to 180 from vertical line it is called plageotropism (leaves, lateral branches, secondary roots). Some organs do not respond to geotropic stimulus and are called ageotropic. e.g. coralloid roots of Cycas.