Plant Growth Process: Phases of Cell Growth and How Seeds Germinate

Plant growth is the process by which new structural elements of an organism are formed.

The structural elements include organs, cells, parts of cells, and even the submicroscopic components of the protoplasm. Plant growth process

What is plant growth?

Plant growth is the formation of new structural elements, usually accompanied by an increase in mass and size. Growth is a property of a living organism, and it is characterized not only by the creation of new structural elements but also by their destruction.

In some cases destruction can prevail over formation or be equal to it, yet this does not mean that growth has stopped. The two processes run side by side throughout the life of the plant.

During the formation of generative organs in cereals and some annual plants, for example, there is no gain in dry matter but rather a loss of it. Even so, growth processes are clearly under way, because new structural elements of the organism are being formed.

When seeds germinate in darkness they produce large seedlings — growth occurs — yet the dry weight of those seedlings is lower than the dry weight of the seeds that were sown. The same loss of weight during growth happens when seeds are sown in soil and when potato tubers are planted.

Until the sprouts reach the surface, obvious growth is taking place while the weight of the plant decreases at the same time. As an organism passes through its life cycle, growth proceeds at varying speeds, and the manner in which new structural elements form changes in a regular way.

What are the phases of cell growth?

Cell growth passes through three successive phases — embryonic, elongation, and differentiation — and the growing zones where these occur lie at the tip of every root and stem. The growing zone is generally small: at a root tip it is about 1.0 cm, while in a stem it ranges from 4.0 to 30.0 cm. At the very end of the growing zone is embryonic tissue, the primary meristem.

The primary meristem consists of identical small cells, completely filled with protoplasm, with thin walls and a large nucleus. From this tissue the cells move through each growth phase in turn.

The embryonic phase of cell growth

In the embryonic phase the cells are in a state of continuous division, while their dimensions stay more or less constant. After each division the mass of living substance in every new cell increases until it reaches the size of the mother cell, after which the cell divides again. The increase in size during this phase is small, because it is determined only by the rising number of cells.

Cells formed in this way then pass into the next phase of growth, which is marked by a substantial increase in the size of the plant cell. Vacuoles appear in the cells and gradually merge into a single large one.

The elongation phase of cell growth

In the elongation phase the amount of protoplasm continues to rise, the cell wall expands, and the cell volume increases sharply. Here the number of cells stays constant; the strong growth is caused by the enlargement of individual cells, linked mainly to the growth of the cell wall and the volume of cell sap.

The differentiation phase of cell growth

In the differentiation phase the cell walls thicken through the layering of hemicelluloses and lignin. The fully grown cells begin to differentiate, forming various tissues; this is why the phase is called differentiation.

During differentiation the cells do not increase in size and their number stays the same, so linear growth is negligible. Once differentiation is complete, growth of the cell ceases. Phases of cell growth

Each cell therefore passes through all three growth phases, occupying in turn the different growth zones, which are arranged in the following order: the zone of embryonic growth at the very tip of the root and stem, then the zone of elongation, and finally the zone of differentiation. These terminal meristems are what allow the root and stem to grow in length. Growth of stems and roots in thickness is possible thanks to a secondary meristem — the cambium.

The cambium occurs only in dicotyledonous plants and lies between the xylem and the phloem. Cambium cells also divide vigorously and pass through all three growth phases, producing xylem and phloem.

Secondary meristem is also present at the base of every leaf, but it usually survives only a short time. In addition, secondary meristem occurs at the base of internodes — the so-called intercalary meristem. The cork cambium is likewise a secondary meristem.

What do cells need in order to grow?

Cell growth requires a supply of organic and mineral substances, water, and the presence of the nucleic acids RNA and DNA, with whose participation the synthesis of protein — the basis of protoplasm — is carried out. In embryonic cells the RNA is found not only in the nucleus but throughout the protoplasm.

As growth continues, RNA becomes concentrated in the parenchyma cells that form, within structural elements such as ribosomes, plastids, and mitochondria. V. G. Konarev showed that the formation of non-protein structures (the rings and spirals of the vessels) during the differentiation phase also takes place with the participation of RNA.

Energy is also essential for plant growth, and it is released during the process of respiration. Beyond energy, the growth processes also require growth activators to proceed.

How can plant growth be observed and measured?

Growth can be observed using a simple method proposed by Sachs: two ink marks are placed 1 mm apart on the embryonic zone of the root or stem of a seedling. The seedling is kept in a moist atmosphere, and each day the section between the marks is measured and its increase recorded in a table.

Following the marked section over several days shows a characteristic pattern. The increase of the section grows at first, then gradually diminishes, and finally the distance between the marks no longer changes — that is, the increase equals zero (see the table below).

Total length and increase of a bean root over eight days

The same kind of data can be obtained within a single day by applying not one but several marks, each 1 mm apart, along the whole growing zone of the seedlings. On the next day all the sections are measured and the increase of each is calculated. Growing zones of stem and root with applied marks:

1. - with marks just applied,
2. - one day later.

The smallest increases occur at the tip of the root or stem, where the cells are in the state of embryonic growth.

The increases rise and reach a maximum in the zone of elongation, then gradually decline toward the end of the zone; in the zone of differentiation no change in the length of the sections is observed.