Plant Physiology - Musiyenko M.M. 2001

Physiology of Plant Growth and Development
Primary Plant Growth

Types of Meristems. Unlike animals, plant growth occurs exclusively in specific regions known as meristems (with the sole exception of early embryonic development stages). Meristems are groups of Cells that retain the capacity for mitotic division. They give rise to the diverse Tissues of the plant body (Fig. 161).

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Fig. 161. Localization of SHOOT and ROOT meristems

There are Three types of meristems: apical, located at the tips of roots and shoots, which provides primary growth, i.e., growth in length. Lateral meristems, or cambium, are found in older parts of plants—for instance, cork cambium (phellogen), which provides Secondary Growth of the periderm, and vascular cambium, which ensures secondary growth, i.e., stem thickening. Finally, the third type is the intercalary meristem, located between certain permanent tissues, such as in the nodes of grasses. It enables growth processes in length within intermediate regions. This type of growth is characteristic of plants whose apical parts are frequently damaged. Apical meristems are characterized by relatively small, cuboidal cells with thin Cellulose walls and dense Cytoplasm. In addition, they contain a significant number of small vacuoles and proplastids. The cells of apical meristems are called initials. During mitotic division, one of the daughter cells remains in the meristem, while the other enlarges and differentiates to become a permanent part of specific plant tissues.

Primary shoot growth. Three main zones of meristematic tissues are distinguished in the shoot: 1 — protoderm, which gives rise to the epidermis; 2 — procambium, which forms conducting tissues, namely the pericycle, phloem, and vascular cambium; 3 — ground meristem, which forms parenchymatous ground tissues. For example, in dicots, these are the cortex and pith.

All these meristematic tissues originate from the division of initial cells at the shoot apex. Growth in length occurs through Cell elongation. At this stage, small vacuoles increase in size and merge into a single large vacuole. Turgor pressure stretches the cellulose fibrils and shapes the final cell type. The total cytoplasmic volume at The final stage remains nearly identical to that of the original meristematic cell, but it is now restricted to The Cell periphery. Upon completion of Cell Formation, wall thickening due to cellulose and hemicellulose is frequently observed.

The procambium forms a series of longitudinal strands consisting of elongated cells. The first cells to differentiate from the procambium are protoxylem cells in the central region and protophloem cells in the outer region. Protoxylem and protophloem elements die off very rapidly,

while their Functions are taken over by the xylem and phloem, which are formed later by the cambium in the differentiation zone. In this zone, each cell specializes to perform its specific function according to its position relative to other cells within the organ. The most dramatic changes occur in the procambial strands, which differentiate into vascular bundles. This is associated with the lignification of sclerenchyma fiber walls and xylem elements, as well as The Development of vessel elements characteristic of xylem vessels and sieve tubes of the phloem. Between the xylem and phloem lie vascular cambium cells that retain The ability to divide, driving secondary thickening.

Shoot development also involves the growth of leaves and lateral buds (Figs. 162, 163). Leaves originate as leaf primordia. They consist of groups of meristematic cells distributed at regular intervals along the shoot. The points of their emergence are called nodes, and the spaces between them are internodes. The plane of Cell Division in the peripheral shoot meristem during leaf initiation is parallel to the apex surface. The arrangement of leaves varies among different species and is termed phyllotaxis. Leaves are positioned to minimize mutual shading (137.5°). Leaf primordia grow very rapidly to form leaf blades. Quite often, after leaves begin to grow, buds form in the axils between the leaves and the stem. These represent dormant meristematic cells that will divide later. Such buds can develop into secondary branches, flowers, or underground structures such as rhizomes and tubers. Axillary buds are believed to be under the control of apical meristems.

Fig. 162. Cytology/practical/54.html">Longitudinal section of the shoot apex: A — photomicrograph, B — diagram

Fig. 163. Woody shoot in autumn-winter (A) and spring-summer (B) periods

Primary root growth. At the very tip of the root apical meristem lies a group of initial meristematic cells that give rise to all other root cells (see Fig. 116). Root cap cells are formed on the outer side. These are relatively large parenchymatous cells that protect the apical meristem and facilitate root penetration into the soil. These cells are continually worn away and die, and are therefore replaced by new ones. In addition to protection, they perform another crucial function: they act as gravity receptors (Fig. 164). They contain starch grains that function as statoliths, settling to the bottom of the cell under METABOLISM/18.html">The Influence of gravity. The zone of cell division spans approximately 1-2 mm from the root tip and slightly overlaps with the zone of cell elongation. The procambium encompasses the entire central cylinder, although in the mature state, it contains the non-conducting tissues of the pericycle and pith (if present).

Fig. 164. Gravity receptors in the root tip: A — geotropic root bending; B — statocytes in the root cap; C — diagram of statocyte Structure

The zone of division is followed by the elongation zone, extending up to 10 mm from the root tip. The elongation of cells in this zone pushes the root tip downward, forcing it to penetrate the soil. Cell Differentiation in the root begins in the division zone with the Development of the first phloem sieve elements. Further away from the root tip, xylem vessels begin to differentiate. Xylem frequently extends all the way to the central axis of the root, in which case no pith is formed. Differentiation concludes when all cells cease elongation. By this time, the development of root hairs from the rhizodermis is also complete.

The taproot can branch, but unlike the shoot, it does not do so via buds. Instead, groups of pericycle cells regain meristematic activity and form a new root apical meristem. It begins to grow, pushing through the endodermis, cortex, and rhizodermis, thereby forming lateral roots.