Tag: biology

• Phototropism is the growth of organisms in response to a stimulus. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. 
• Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth.
  Hence, option A is correct.

Medulla oblongata: This section of the brain helps transfer messages to the spinal cord and the thalamus in the brain from the body and controls breathing, heart function, blood vessel function, digestion, sneezing, and swallowing. Sensory and motor neurons from the forebrain and midbrain travel through the medulla.The hypothalamus is a section of the brain responsible for hormone production. The hormones produced by this area of the brain govern body temperature, thirst, hunger, sleep, circadian rhythm, moods, sex drive, and the release of other hormones in the body.
The cerebrum or cortex is the largest part of the human brain, associated with higher brain function such as thought and action. The cerebral cortex is divided into four sections, called "lobes": the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.
The diencephalon is the region of the embryonic vertebrate neural tube that gives rise to posterior forebrain structures including the thalamus, hypothalamus, posterior portion of the pituitary gland, and pineal gland.
Chemo-tropism is the growth of organisms (or parts of an organism, including individual cells) such as bacteria and plants, navigated by chemical stimulus from outside of the organism or organisms part. The response of the organism or organism part is termed positive if the growth is towards the stimulus or negative if the growth is away from the stimulus.
An example of chemo-tropic movement can be seen during the growth of the pollen tube, where growth is always towards the ovules. It can be also written that conversion of flower into fruit is the example of chemo-tropism.

Tendrils are the threadlike organs of plants which are actually specialized stem, leaf or petiole and used for climbing, support or attaching to another for parasitic growth. Most climbing plants such as vines develop tendrils that tend to coil around objects that support it. This feature of the tendrils is due to a phenomenon called thigmotropism - directional growth movement of curvature in response to stimulus or contact.

Auxin is a plant hormone that plays important role in the growth of stem and roots to gravity. Gravitropism is a plant growth response to gravity. Root shows positive gravitropism and shoot shows negative gravitropism. Auxin content is lower on the upper side and more concentration occurs in lower side of the root and stem. This stimulates stem elongation and thus stem grows upwards. In roots, higher growth on upper side occurs due to lower auxin content. This inhibits root elongation and thus root grows downwards showing geotropic response. 

Auxin is a plant hormone that plays important role in the growth of stem and roots to gravity. Gravitropism is a plant growth response to gravity. Root shows positive gravitropism and shoot shows negative gravitropism. When plant is placed horizontally, more concentration occurs in lower side of the root and stem. This stimulates stem elongation and thus stem grows upwards. This inhibits root elongation and thus root grows downwards. 

Turgor movement is a reversible change in position of a plant part due to a change in turgidity of cells. It is mostly caused by contact. Mimosa pudica shows dropping of leaves upon touching. In this sensitive plant, the pulvinus and pulvinnules are responsible for the folding of the leaves. In this, turgor changes occur in thin walled cells of pulvinus present on lower side and pulvinnules present on upper side. This results in folding of pinnules and pinnae comes together causing drooping of the compound leaves.

Nutation is the bending movements shown by some plant parts due to the unequal rate of growth of different parts of plants. Thigmonasty is the nastic response of a plant to touch. The movements of tentacles of Drosera when comes in contact with insect is due to thigmonasty. The movement of plant part in response to contact with a solid touch is called as thigmotropism. The curvature movements due to support seen in tendrils and twiners is an example of thigmotropism. The movement of parts of a plant in response to air current is called as anemotropism. 

Phototaxis is a movement of part of organism in response to the light. This phenomenon is exhibited by zoospores of certain algae like Ulothrix, Chlamydomonas, Volvox, Cladophora. They exhibit positive phototaxic movement in presence of diffused light and negative phototaxic movement in presence of intense light.

Nutation is the bending movements shown by some plant parts like root, stem, leaf due to the unequal rate of growth of different parts of plants.