Why Do Two Plants From the Same Field Grow So Differently?

You know from Grade 8 how a plant starts: a seed takes in water, gets air and warmth, and its embryo wakes up and germinates Grade 8: Factors affecting germination. That is only the first few days. After that, the seedling has to grow for weeks or months, into a plant that can flower and make seeds of its own.

To grow, a plant needs two things:

• Food, which it makes itself. You learned in Grade 6 that a green plant makes food by photosynthesis: in sunlight, its leaves join carbon dioxide from the air with water from the soil, to make sugar. The green chlorophyll in its chloroplasts catches the light Grade 8: Plastids. Almost everything a plant is made of was built from that sugar.
• Nutrients from the soil: nitrogen, phosphorus, potassium and the other elements a plant needs in small amounts, which you will meet in Grade 9 Grade 9: Necessary nutrients for plants.

With them, the plant makes new cells at the tips of its shoots and roots, and the cells grow bigger. That is growth: a lasting increase in the size and mass of a living thing.

How to measure growth

You can measure a plant’s growth in several ways:

• a.its height, with a ruler;
• b.the number of leaves, or the size of the biggest leaf;
• c.its mass, on a scale;
• d.for a crop, the yield: how much grain or fruit it gives.

Height is the easiest, but it can fool you, as you will see. Mass is the best measure of how much a plant has really grown. Scientists often dry the plant in an oven first and weigh its dry mass, because the amount of water in a plant changes from hour to hour.

Two kinds of cause

Everything that affects how a plant grows belongs to one of two groups.

• Environmental factors are the plant’s surroundings: how much light, water, nutrients, space and warmth it gets. The soil.
• Genetic factors are what the plant inherited from its parents, in its seed: what kind of plant it is (a species, like maize or grass) and which variety of that kind (a tall local maize or a short new one). The seed. For animals, the same thing is a breed: a Jersey cow or a local cow.

You will learn in Grade 10 how characters are passed on from parents, in genes Grade 10: Heredity and Mendelism. Here you don’t need to know how genes work: only that the seed carries something from the parents that decides what kind of plant it can become.

Light

Every year at Dashain, families sow barley or maize in a dark room on the first day, Ghatasthapana. By the tenth day the shoots of jamara are a hand long, and they are yellow, not green. Why?

A shoot grown in the dark has no light to make chlorophyll with, so it stays pale yellow. And it grows tall and thin, fast, as if it is stretching to find light. A seedling growing under a stone or deep in the soil does just that: it pushes up quickly until it reaches the light, and only then opens its leaves and turns green. Growing tall, thin and pale in the dark is called etiolation.

So is the dark-grown seedling growing better, because it is taller? No. It is living on the food stored in its seed, and it can’t make any more without light. Weigh the two after drying, and the one in the light is heavier: it has built more. Leave the dark one in the dark, and it dies when its seed’s food runs out. This is why height alone can fool you.

Water

In a dry spell, the leaves of maize roll up and droop. A plant short of water closes the tiny pores in its leaves to save water. But those are the pores that let carbon dioxide in, so with them closed, photosynthesis slows down, and the plant makes less food. Short of water for long, it grows less, flowers late, and gives less grain. That is why a late or weak monsoon means a smaller rice harvest. And water can be too much: you saw in Grade 8 that seeds under water rot for lack of air, and the roots of most crops (not rice) suffer the same in waterlogged soil.

Nutrients from the soil

In Grade 9 you will meet Harka Bahadur, whose maize grew unevenly until he tested his soil and gave each part of his field the fertilizer it lacked Grade 9: Harka Bahadur’s maize. A plant short of nitrogen stays small and its lower leaves turn yellow; short of phosphorus, its roots stay weak. Fertilizer or compost puts back what the soil lacks, and the plant grows more. But only up to a point, as you will see below.

Space

Sow mustard seeds thickly in one corner of a bed, and you get a crowd of thin, spindly seedlings. That is why farmers thin out radish and mustard seedlings, and transplant rice seedlings in rows, a hand’s width or more apart. Plants packed close together compete: their leaves shade each other, and their roots share the same water and nutrients. Each one gets less, so each one grows less.

The shortest stave

A farmer’s maize is short of water. She spreads more urea. Will it help? Probably not much. A plant can only grow as fast as its scarcest need allows. If it is short of water, extra nitrogen won’t make it grow; it is held back by the water. The need in shortest supply is the limiting factor. Scientists in the 1800s called this the law of the minimum. It is often drawn as a barrel made of wooden staves: water poured in only rises to the top of the shortest stave, however tall the others are.

Real plants don’t follow the barrel exactly: a plant short of one thing can sometimes make up a little with another. But the lesson is true and useful. If you want a plant to grow more, first find what it is most short of. A farmer who spreads fertilizer on a field that is really short of water wastes her money.

Now turn it round. Put different seeds in the same surroundings. Do they grow the same?

Different kinds of plant

On the edge of the same field you may see two grasses side by side. Dubo, the grass offered at puja, creeps along the ground and its upright shoots rarely stand more than a hand or two high. Napier grass, grown for fodder, stands 2 to 4 meters tall, much taller than a person. Same soil, same rain, same sun. They grow so differently because they are different species: each inherited its way of growing from its parents.

Different varieties

Within one kind of crop, there are many varieties, each bred from parents chosen for certain qualities. Rice in Nepal grows from the hot Terai at about 60 m above sea level up to the cold valleys of Jumla. There, a red rice called Jumli Marshi is grown up to about 3,050 m, said to be the highest rice fields in the world. Most rice varieties would hardly grow in Jumla’s cold. Jumli Marshi has inherited the ability to grow in it.

Different breeds

Animals are the same. High in Mustang and Manang lives the Lulu, a dwarf cow. A grown Lulu cow is under 90 cm tall at the shoulder and weighs about 70–150 kg. A grown Holstein Friesian cow, the black-and-white dairy breed, stands about 1.5 m tall and weighs about 700 kg, around five times as much as a big Lulu. Even fed well, a Lulu calf won’t grow into a Holstein-sized cow. Its size is in its genes.

Copies with the same genes

There is a neat way to do the opposite: keep the genes the same and change the surroundings. You learned in Grade 8 that a potato tuber, a sugarcane cutting or a grafted branch grows into a plant just like its parent Grade 8: Vegetative propagation. Those plants have exactly the same genes as the parent, and as each other. So if you plant pieces of one potato in a sunny bed and a shady one, any difference in how they grow comes from the surroundings.

So which matters more, the seed or the soil? The real answer is that they work together. The genes set what a plant or animal can become. The surroundings decide how much of that it actually reaches.

A broiler chick has the genes to reach 4 kg in eight weeks, but only if it gets rich feed, water and warmth. Let it peck for scraps in a yard, and it grows far less. A Lulu calf given the best feed in the world still grows into a small cow. Good genes need good surroundings to show what they can do, and good surroundings can only bring out what the genes allow.

Sometimes one variety is better in one place and another variety is better somewhere else. Which seed is “best” depends on the field.

This is the clearest kind of evidence that genes and surroundings work together. You can’t say “fertilizer increases yield”, because it depends on the variety. You can’t say “the short variety is better”, because with no fertilizer, the two are close. The effect of one factor depends on the other.

Short rice and wheat, together with fertilizer and irrigation, about doubled Asia’s harvests of grain between 1970 and 1995. That change was called the Green Revolution. It had costs too: farmers came to depend on bought seed and fertilizer, and many old local varieties stopped being grown.

Animals and people too

Fish in a pond show the same thing. Fish farmers in Chitwan know that if they put too many young carp in a pond, each fish ends up small: there is only so much food and oxygen in the water to go round. The same young fish, fewer to a pond, grow much bigger.

And people. Tall parents tend to have tall children: height is partly in the genes. But food and health matter enormously. In 2016 a team of scientists collected the heights of more than 18 million people around the world. Women in South Korea born in 1996 grew up about 20 cm taller than women born there in 1896. Genes don’t change in a hundred years; food, clean water and health care did.

In Nepal, as in many countries, many young children grow less tall than they could, because of too little good food and too much illness, especially in the first two years. This is getting better, thanks to better food, health care and clean water, not to any change in genes.

How do scientists find out whether something affects growth? They do a fair test:

• a.change one factor only (the one you are testing), such as fertilizer;
• b.keep everything else the same: the seed, the soil, the pot, the water, the light, the days;
• c.use many plants for each, not one, because no two plants grow exactly alike;
• d.measure each plant, and work out the average.

Why many plants? One plant might be small because its seed was damaged, or a snail ate a leaf. With ten plants, the odd one makes little difference to the average.

Claim, evidence, reasoning

When a scientist explains something, she gives three things:

1. a claim: what she thinks is true (“Compost made the mustard grow more”);
2. the evidence: the measurements that support it (“The plants with compost were 15 cm tall on average, those without 10 cm”);
3. the reasoning: why the evidence supports the claim (“Both trays had the same seed, water, light and days, so the only difference was the compost. Compost adds nutrients, which plants need to grow”).

Try to give all three whenever you explain something in science.

• How Did Farmers Make Our Crops? How farmers and scientists choose parent plants to breed varieties like IR8, Jumli Marshi and Sukkha Dhan, over thousands of years.
• Why Do You Look Like Your Parents, but Not Exactly? What genes are, and how they pass from parents to their young, so that each young one is a little different.
• What Happens to the Dal Bhat You Eat? Inside your body, the food you eat becomes energy and new cells: how you grow.