Growth of Plants

C. H. Wadleigh.

Whatever any plant does during the course of its growth cycle depends on its hereditary background and the environment under which it is grown.

The best of soil-management practices cannot make up for inferior germ plasm the substance by which hereditary characters are transmitted in our crops. You have to grow the varieties with the most desirable capabilities for your climate and soil conditions. Soil management cannot make a silk purse out of a sow's ear.

Consider a few illustrations: Varieties of sugar beets that do not carry resistance to curly top disease are apt to be a total loss when they are grown in many of the Western States, regardless of cultural practices.

Even the best farmers in Minnesota cannot produce a profitable crop of wheat if they plant seed that does not carry resistance to a prevailing strain of stem rust.

California Common alfalfa grows luxuriantly in the Imperial Valley during the fall, but a northern variety like Ranger grown there at that time makes little growth because of its sensitivity to the length of day. The growth of Ranger practically stops in the fall with the onset of short days regardless of how favorable the weather and soil conditions may be.

Even though a plant has a good hereditary background for the most desired type of productivity, its potentialities cannot be reached fully in an adverse environment.

Cultural operations on the soil affect plant growth through their influence on the environment of an exceedingly important part of the plant the roots. Even so, favorable conditions around the roots will be limited in their effects by the nature of aerial environment.

Why do plants grow?

By what processes and mechanisms do conditions about the tops and roots of plants influence growth?

Such questions cannot be answered fully. The secret of life is still a secret.

Yet progress in biological research constantly is increasing our understanding of why and how the wheels go 'round inside a living cell and why and how external conditions affect this "machinery."

GROWTH of an annual seed plant is in four stages: Seed germination, vegetative development, reproductive processes, and the maturing of the seed.

A mature seed contains a small, living, quiescent plant. This miniature plant is the embryo, or germ. When conditions of temperature, moisture, and oxygen are right, the embryo starts to grow. This process, including the bursting of the seedcoat, is germination. When the embryo plant has developed enough to lead an independent life without drawing on the nourishment stored in the seed, germination is complete.

Absorption of water is the first stage in germination. The rate at which seeds absorb water from the soil depends mainly on the moisture in the soil, the compactness of the soil around the seed, temperature, and the nature of the seedcoat. If the water content of the soil is down to the permanent wilting percentage, seeds cannot absorb enough water to germinate. Loose soil around the seed may prevent contact with a sufficient number of moisture-coated soil particles to transfer the amount of water needed for germination.

But excessive compaction of a moist soil may limit absorption of water by limiting the supply of highly essential oxygen. Thus the press wheel on seed planters should bring moist soil particles in close contact with the seed without too much compaction.

Seeds absorb water faster from a warm soil than from a cold one.

Some seeds have hard coats that water will not penetrate. They may need special treatment, such as scarifying (scratching the seedcoat) to make them more penetrable to water. Some seeds like apple, rose, and iris will not germinate even when the moisture supply is favorable unless they go through a series of changes known as afterripening--a process that usually involves exposure to low temperatures in a moist environment. Some seeds, like Grand Rapids lettuce, require exposure to light in the presence of moisture in order to germinate.

The second stage in germination is the digestion of stored nutrients and the transfer of these solubilized nutrients to the young plant or embryo. As water enters the seed, the seed coat is softened, facilitating the entry of oxygen and the release of carbon dioxide (CO₂), given off by respiration of the awakening seed.

With the absorption of water, the digestive compounds enzymes in the seed become active and digest or break down the complex stored foods, such as fats, starches, and proteins, into soluble forms. They are translocated to the embryo and are used in the formation and growth of the new cells that comprise the first shoot and root as they emerge from the seed.

These processes. in the germinating seed are greatly affected by temperature. Some seeds will not germinate when the temperature of the soil is below 40 to 50 F. There is no specific optimum temperature for seed germination. The most suitable temperatures vary with the kind of plant; 65 to 80 are often favorable, depending on the kind of seed. Above 100 , germination is impeded. Therefore soil-management practices that aid in warming the soil in cool seasons may help seeds to germinate.

The digestion of stored foods in seeds and their reutilization in the growing embryo require relatively high levels of oxygen. Large amounts of CO₂ are given off. Facility for gaseous interchange around germinating seeds is highly important. That is why compacted soils or soggy, wet soils hinder or prevent germination.

As the young seedling pushes its way upward through the soil, mechanical work is done in thrusting soil particles aside and breaking any crust that may be present on the surface. When soils are seriously encrusted, more force may be required for a breakthrough than the little seedlings can muster, and they perish in the effort.

Cold, wet soils seem to foster the activities of the many kinds of disease organisms that are ready to attack weakened seedlings and thus cause serious agricultural loss.

Growers of cotton, sugar beets, soybeans, and other row crops know that one of their stiffest hurdles in producing a crop is getting a stand. In other words, attaining soil conditions that will provide optimum seed germination regardless of weather is important in getting high yields.

The little seedling, as it pops through the soil, starts on a new phase of development. It no longer depends on nutrient reserves in the seed. Its continued life and growth depend on its ability to make and use its own food supplies. The period of growth following seedling emergence and on to flowering is vegetative development.

Let us examine just what is meant by this word "growth" when applied to plants.

Growth consists of an irreversible increase in size. This usually (but not always) includes an increase in dry matter. Growth usually is accompanied by a change in form, shape, or state of complexity, which constitute the processes of development. Growth is measurable by a balance or a rule; development is characterized by descriptive adjectives.

How does growth take place?

Each crop plant is made up of billions of minute cubicles, the cells. A plant increases in size as the cells increase in number or size or both. If our cabbages stop growing, it means that the cells making up the leaves of the plants have stopped increasing in number and size.

The cells that provide the increases in number are only in certain parts of the plants the meristems. They are the cambial layers (familiar as the "slipping" areas between the bark and wood of trees), the growing points of stem and root tips, and the blades of young leaves. Meristem cells can multiply by division. At a certain stage of their growth, the various contents of the cells are divided into two parts, and a new cell wall is laid down between the parts.

Some of the newly formed cells enlarge enormously with intake of water.

Other cells are destined to develop openings in consecutive ends to form the elaborate conductive system, or "plumbing," that is always present in the crop plants. Still others become strangely elongated and form thick walls, which are the fiber and wood cells that give rigidity to the framework of the plant.

Why do cells grow and thereby bring about the growth of our corn, cotton, or cabbage plants?

We do not know. We do know some of the essentials.

Every living cell contains a complex proteinlike substance, somewhat like egg white, called protoplasm. It is the physical basis of life.

Protoplasm contains fatty substances in addition to proteins. It is 80 to 85 percent water. It is distributed over the inner wall of the cell cubicle. Water fills the interior of a living cell,

and this contains a host of substances, as sugars, amino acids, hormones, organic acids, enzymes, minerals, and more complex materials that are essential for the life and function of the protoplasm.

An essential part of our discussion is how environment affects the components and thereby the life and growth of the cells that make up the growth of a plant.

Increase in size of a cell means that work was done and energy was utilized.

The energy comes from sunlight. Light energy from the sun is absorbed by chlorophyll the green color in leaves which in turn converts the CO₂ absorbed by the leaves and the water absorbed by the roots to simple sugars, a process called photosynthesis. The simple sugars are depositories of energy from the sun. They are the raw materials for the plant's diverse chemical syntheses and growth processes.

Thus light is essential for growth. A grower of greenhouse tomatoes in Cleveland knows that he can expect only half the production per plant from his fall crop as compared to his spring crop. The difference in production is merely a simple reflection of the difference in the light energy available to the tomato plants at the two seasons.

Carbon dioxide also is essential for growth. The atmosphere normally contains only 0.03 percent of CO₂. Under high light intensities during the summer, this low level of CO₂ in the air probably is the main limiting factor in the photosynthesis.