Ch 7Plant Growth and Mineral Nutrition — Class 12 Biology, concept-first.

This chapter follows growth from the dividing cell to the whole plant - its phases, how it is measured, the hormones that regulate it, the light and temperature cues that switch a plant to flowering -…

Growth is one of the defining features of every living organism, and in plants it shows two related aspects.

The dividing, enlarging and differentiating cells of a meristem pass through three overlapping phases of growth.

Growth in plants can be measured in terms of....

Growth in plants depends on a combination of environmental and physiological conditions acting together.

Growth rate : The growth rate is the amount of growth occurring per unit time; it is also called the efficiency index, and can be measured as an increase in the size or area of a leaf, flower or fruit…

A growth curve is a graphic representation of the total growth against time. There are three types of curves: linear, exponential and sigmoid.

a. Differentiation : Cells derived from the apical meristem of the root or shoot mature into their final, functional form.

Development refers to all the ordered, progressive changes in the shape, form and complexity of a plant across its life cycle -- from the germination of the seed to eventual senescence and death.

Plasticity is the capacity of a plant to be moulded or modelled -- its ability to produce different kinds of structures in response to different external (environmental) or internal stimuli at differe…

The word 'hormone' was first coined by Starling in 1906, in the context of animal physiology. In plants, the internal chemical factors that influence growth -- by inhibiting, promoting or otherwise mo…

a. Auxins (Auxien = to grow) : F. W. Went first used the term auxin in 1931; auxin was chemically isolated that same year, from the urine of a person suffering from pellagra, by Kogl and Haagen-Smit.

b. Gibberellins : Gibberellins are another class of growth-promoting hormones, particularly abundant in root tips and developing seeds; unlike auxin, they are transported in a non-polar fashion throug…

c. Cytokinin : Cytokinins are growth hormones that specifically promote cell division; the name was coined by Letham.

d. Ethylene : Ethylene is the only gaseous plant growth regulator. Denny reported in 1924 that ethylene was effective in ripening fruit; Gane established in 1934 that plants themselves naturally synth…

e. Abscissic Acid : Abscisic acid (ABA) is a naturally occurring growth-inhibiting hormone. Carns and Addicott (1961-65) found that the shedding of cotton bolls was caused by chemical substances they…

Higher plants reproduce sexually by producing flowers, and the shift from vegetative to reproductive growth -- flowering -- is, like vegetative growth itself, controlled by environmental and nutrition…

a. Short Day Plants (SDP) : Short Day Plants (SDP) typically flower in winter or late summer, when the length of the day is shorter than a critical photoperiod -- the specific day length above or belo…

b. Long Day Plants (LDP) : Long Day Plants (LDP) typically flower during summer, and require a period of light longer than the critical photoperiod in order to flower.

c. Day Neutral Plants (DNP) : Day Neutral Plants (DNP) flower throughout the year, independent of day length -- they have no specific photoperiod requirement for flowering, which is why they are calle…

Phytochrome : Hendricks and Borthwick (1952) found that flowering in short-day plants is inhibited if the long dark period is interrupted by even a brief flash of red light (660 nm) -- but that if thi…

Temperature, besides light, is the other major environmental factor governing flowering; a plant's response to temperature is described by three cardinal points -- minimum, optimum and maximum.

Besides water and gases, plants absorb minerals and nutrients from their surroundings. Green plants need a range of inorganic substances -- obtained from the soil as minerals -- to synthesise their or…

Classification of minerals : Minerals were traditionally classified, on the basis of whether the plant absolutely required them, into essential and non-essential elements.

Symptoms of Mineral deficiency in plants : Any visible deviation of a plant from its normal structure or function is called a symptom, or 'hunger sign', of mineral deficiency.

Toxicity of Micronutrients : A mineral ion is considered toxic when its concentration in the tissue reduces dry weight by 10% or more.

Most minerals in the soil exist as charged particles (ions), and charged particles cannot simply diffuse across a cell membrane, so the majority of mineral uptake by roots requires an active, energy-c…

The nitrogen cycle is the series of natural processes by which nitrogen moves from the atmosphere into living organisms, through the soil, and back again to the environment.

Nitrogen fixation : The atmosphere is the source of nitrogen, but it cannot be used directly - it combines with C, H, N and O to form compounds before being used.

Nitrification : Nitrification is an exothermic reaction. Most of the soil bacteria participate in converting ammonia into nitrate, the form of nitrogen that can be used by plants and animals.

Nitrogen assimilation : In the soil nitrogen is present as nitrates, nitrites and ammonia (NH4+). It is absorbed by green plants and converted into nitrogenous organic compounds like amino acids and D…

Denitrification : It is the process by which anaerobic bacteria in the soil convert nitrates back into gaseous nitrogen, removing fixed nitrogen from the ecosystem and returning it to the atmosphere i…

Q.1 of the textbook exercise (p151): twelve multiple-choice questions.

Q.2 of the textbook exercise (p151): ten very-short-answer questions.

Q.3 of the textbook exercise (p152): five short-answer questions; question 1 has two parts (a, b) and question 3 has three (a, b, c).

Q.4 of the textbook exercise (p152): five long-answer questions.

The textbook's end-of-chapter Project (p152): four tasks on germination, viviparous germination and mineral-deficiency symptoms. A guided approach is given for each.

Sample papers and previous-year board questions for this subject.