Chapter 15: Plant Growth and Development

Ans: Plants that need to be exposed to light for a shorter length of time than the crucial light period are known as short day plants. Only when the photoperiod is below the critical period, i.e. when there is a continuous extended dark period, does this plant blossom. These plants do not develop blooms when exposed to a photoperiod that is longer than the crucial period. In plants, a hormone causes flowering. In response to the needed photoperiod, the hormone is found in leaves and migrates to the shoot apices.

Ans: Meristematic cells around the root tip are actively dividing. The cells are characterised by thin walls, thick cytoplasm, and a big nucleus.

Ans: The cells in this zone have a high number of tiny vacuoles. These vacuoles join together to create a single big centre vacuole. As a result, cells in this area become stretched and cause longitudinal growth.

Ans: Plants grow in an unpredictably unexpected way, but mammals grow in a predictable way. This means that the bulk of plant components will keep growing eternally. However, this does not imply that all plant components continue to grow indefinitely. Only a few plant components, such as the root and shoot apexes, can grow endlessly.

Ans: When meristematic cells become specialised for a particular function, they stop dividing. This phenomenon is called differentiation. During differentiation, cells undergo major structural changes in protoplasm and cell wall.

Ans: The living differentiated cells can regain the capability of division under suitable conditions. This phenomenon is called de-differentiation. For example; formation of meristems from fully developed parenchyma happens because of de-differentiation.

Ans: When de-differentiated cells become mature and assume specialized function once again, this is called redifferentiation. For example; cambium redifferentiates to form the cortex.

Ans: The raw materials required for initial synthesis of auxins are called auxin precursors.

Ans: Anti-auxins, such as p– chlorophenoxy isobutyric acid (PCIB) and TIBA, are compounds that block the activity of auxins.

Ans: Auxins which are made artificially are called synthetic auxins, e. g. 2,4-D, NAA.

Ans: Following examples show that ethylene and abscissic acid have both positive and negative roles.

Positive Effects of Ethylene: Promotes horizontal growth. Promotes positive geotropism in stem. Promotes apical dominance. Breaks seed and bud dormancy. Promotes fruit ripening and root initiation.

Negative Effects of Ethylene: Inhibits longitudinal growth. Enhances senescence of leaves, flowers and fruits. Prolongs dormancy of lateral buds.

Positive Effects of Abscissic Acid: Helps in reducing transpiration and thus in preventing water loss. Induces root initiation and flowering in some plants. Promotes seed dormancy and bud dormancy. Helps plants develop resistance against cold and other types of stress.

Negative Effects of Abscissic Acid: Promotes senescence of leaves. Retards cambial activity. Inhibits the function of growth hormones. Inhibits seed germination.

Ans: Plant hormones often work in combination. Some plant hormones may have antagonistic effects, while some others may have augmentative effects. It is almost impossible to pinpoint any single hormone for a particular effect in a plant part. Moreover, plant hormones bring out their effect in coordination with various extrinsic factors. Hence, it is difficult to assign any effect seen to any kind of hormone.

Ans: This phenomenon is called apical dominance. The growing shoot apex inhibits the growth of lateral branches. This happens because of a high concentration of auxins in shoot apex. To prevent apical dominance, shoot apex is trimmed. This helps auxins to move to other meristematic regions and to promote lateral branching.

Ans: Plant hormone's are produced by individual cells; in regions where they are required by the plants. For example; auxin is synthesize in the cells of growing shoot apices. Plant hormones are translocated through the vascular system of plants. For example; auxin from the shoot apex may get mobilized to the lateral buds or flower buds. Similarly, some plant hormones may be mobilized to senescent parts.

Ans: This is true that the plant hormones have been discovered accidentally. For example; auxin was accidentally discovered by Charles Darwin and his son Francis Darwin. They observed that coleoptiles of canary grass responded to unilateral illumination by growing towards the source of light. Auxin was later isolated from coleoptiles of oat seedlings by F W Went. The term 'serendipity' is used for accidental discovery.