Q.List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Plant Growth: From Seed to Tree

Think about a tiny mustard seed. It's hard, dry, and seems lifeless. Yet put it in moist soil with some warmth, and within days a small root pushes down and a shoot pushes up. Months later, that same seed has become a plant with dozens of leaves, a thick stem, and perhaps flowers and fruits. Where did all that new material come from? How does a plant "know" where to grow?

The answer lies in a few simple but powerful ideas.

The Intuition: Growth is Local and Permanent

Unlike animals, whose bodies grow everywhere at once (a baby becomes a toddler, and every part gets bigger), plants grow only at specific growth zones called meristems. These are like tiny factories of actively dividing cells located at the tips of roots and shoots, and in a ring inside stems and roots of woody plants.

Growth in plants is also irreversible. Once a cell has expanded and matured, it doesn't shrink back. A leaf that has unfurled stays that size; a stem that has elongated stays that length. This is different from, say, a muscle that can contract or a balloon that can deflate.

So plant growth = cell division (making more cells) + cell enlargement (making those cells bigger) + cell differentiation (cells becoming specialized, like xylem or phloem).

The Precise Statement: Phases of Growth

In biology, plant growth is formally described in three sequential phases:

1. Meristematic Phase – Cells in the meristem are small, thin-walled, with dense cytoplasm and large nuclei. They divide actively (mitosis). This is the formative stage.

2. Elongation Phase – Just behind the meristem, cells stop dividing and start absorbing water. Their vacuoles enlarge, pushing the cell wall outward. This is where the actual increase in length happens — the plant gets taller, roots go deeper.

3. Maturation Phase – Cells reach their final size and shape. They differentiate into specific tissues: some become xylem (water-conducting), some become phloem (food-conducting), some become epidermis (protective layer), etc. Growth stops here for that cell.

Measuring Growth: The Curve

If you plot the size (height, weight, leaf area) of a plant against time, you rarely get a straight line. Instead, you get an S-shaped curve (sigmoid curve):

• Lag phase – Slow growth initially, as the seed germinates and roots establish.
• Log/exponential phase – Rapid growth, as leaves capture sunlight and roots absorb water and minerals. The plant is making more and more food, fueling faster growth.
• Stationary phase – Growth slows and eventually stops, as the plant reaches maturity, flowers, and fruits. Resources are diverted to reproduction.

The Arithmetic of Growth: Geometric vs Arithmetic

A classic exam point: geometric growth vs arithmetic growth.

• Arithmetic growth – Only one daughter cell continues to divide after mitosis; the other matures. This happens in roots and shoots at the very tip. The increase per unit time is constant. Example: root elongation in a young seedling — it grows, say, 2 mm every day, day after day.

• Geometric growth – Both daughter cells continue to divide. This happens in early embryo development and in young leaves. The number of cells doubles each division cycle: 1 → 2 → 4 → 8 → 16 → ... This is explosive growth, but it cannot continue indefinitely because resources are limited. …