Plant Physiology - Musienko, M. M. 2001

Physiology of Plant Growth and Development
Types of Growth

Different plant Organs exhibit various types of growth, which is determined primarily by the Location OF THE growth cone (meristem). For example, in stems and roots, the growth cone occupies a so-called terminal position, where both the cone itself and the newly formed Cells constitute the morphologically upper part of the organ. Consequently, both roots and stems grow from their tips. This type of growth is called apical growth. However, while the growth cone in above-ground organs occupies a fairly extensive zone, active growth in roots is localized within a much more restricted area. In stems, the growth zone can be 2–30 cm or more in length, whereas in roots it is a mere 10 mm, although in aerial roots it may reach up to 100 mm. This specific feature of ROOT growth has high adaptive significance, as it ensures more successful penetration and movement of the root through the soil. Individual sections of the root growth zone grow unevenly; furthermore, the growth pattern of the same organ can vary depending on the plant species.

Thus, in grasses, stem (culm) growth occurs at the Base of the internode. This type of active growth zone located between already formed Tissues that have completed their GROWTH AND DEVELOPMENT is called intercalary growth. The intercalary meristem located at the base of the internode in grasses remains active for a long time. This explains the ability of lodged cereal crops to stand back up even when lodging occurs at late stages of plant development.

Another distinguished type is basal growth, where the growth zone is located at the base of the organ, while the tissues that have completed their growth lie above the growth zone. This type of growth occurs in the leaves of grasses, herbs, and other monocots, as well as in floral scapes.

A different growth pattern is characteristic of the leaves of many dicots, such as tobacco, where growth takes place across the entire periphery of the leaf at a nearly uniform rate in all its parts.

Secondary thickness growth occurs through The activity of secondary Meristems. Cambial cells divide in the tangential direction, As a result of which they are always arranged in regular radial layers. Cambial division produces xylem and phloem elements, with the number of generated xylem elements significantly exceeding that of phloem. Parenchyma Cells of the medullary rays, which separate the cambial bundles, are also activated under METABOLISM/18.html">The Influence of fascicular cambium: they begin to produce stem parenchyma cells. This is the General scheme of stem and root thickening.

Plant growth intensity can be determined by measuring length, volume, fresh weight, and dry weight. Special instruments are used to monitor these processes. Growth rate is characterized by several parameters: specific growth rate is the increase in plant biomass per unit of time:

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where W₀ is the initial dry mass; — the final dry mass; t is the vegetation period. Absolute growth rate is the increment over a certain period of time:

where W₀ is the initial dimensions; W₁ is the dimensions at a specific time.

Relative growth (increment) calculated as a percentage of the initial mass of the plant or organ.

Growth occurs at various Levels of biological Organization—from THE MOLECULAR LEVEL to population coenoses. In all cases, depending on time, growth is expressed by an S-shaped curve (Fig. 165).

Fig. 165. Sigmoidal plant growth curve

This curve is called the sigmoidal curve and can be divided into four sections. The initial phase, or lag phase, is characterized by negligible growth. It then transitions into the logarithmic phase of intensive growth, where growth proceeds exponentially. At this point, the growth rate reaches its maximum, and the number of cells or organisms is highest. Thereafter, the growth rate declines. The point at which this occurs is called the inflection point. The third phase is the slowed growth phase. Finally, the concluding phase is the plateau phase, or stationary growth phase. During this period, overall growth is complete, and all parameters remain constant. In some cases, a slight upward trend in the growth curve may continue until the Organism dies (as observed, for example, in monocot leaves).

Growth can be measured at the cellular, organismal, and whole-coenosis levels. At the organism level, various parameters can be measured: length, surface area, volume, and mass. In plants, it is often necessary to obtain the growth curve of a root, stem, internode, or leaf surface. Mass can be assessed using fresh and dry matter. The main drawback of using fresh weight as an indicator of growth is that the resulting data can fluctuate depending on the moisture content within the tissue. True growth is reflected in changes of other indicators rather than fresh weight, and therefore it can only be measured by the dry matter determination method. It should be borne in mind that in the broad sense of the word, growth is not merely an increase in dry matter as a result of Cell Division, but also the subsequent developmental process.