Plant growth
Plant growth is the increase in the size of a plant .
General
Unlike most animals, plants do not grow everywhere at the same time, but only in certain places. In addition, the growth of most plants is not limited, but can continue for a very long time. The giant sequoia does reach sexual maturity after a few years, but continues to grow and can be up to 3,000 years old and 100 meters high. This is based on the fact that the actively dividing tissues, the meristems , from which growth starts, continue to grow. However, some plant organs show limited growth, such as the leaves , thorns and flowers . In addition, many plants have periods of rest in which they do not or hardly grow.
Division growth and elongation growth
In a meristem, the cells multiply through cell division . In some of the daughter cells, this division growth changes into elongation growth , in which the volume of the cell increases. The cell can expand approximately uniformly (isodiametrically) in all directions, or the expansion can be directed, which can result in very elongated cells. The elongation is based on the uptake of water into the vacuoles , which can eventually fuse to form a central vacuole, while the cytoplasm does not have to multiply and is restricted to the periphery of the cell. A special form of elongation growth is the tip growth of the root hairs and the pollen tubes , in which only the tip extends. Likewise, the hyphae of the mushrooms (which are no longer considered to be plants today) only grow at the tip (apical).
Primary and secondary growth
The meristems of the shoot tip and the root tip ( apical meristems ) are already created in the embryo . The primary growth comes from them. In trees in particular, there is also a secondary growth in thickness that begins later and originates from the cambium , a meristem located in and between the vascular bundles , which represents a cylinder that runs through the entire stem axis and root and forms the wood inward . In addition at the shoot axis, the further outward cork cambium , the outwardly cork forms.
branch
The primary growth also includes branching, which occurs in very different ways in the shoot and root. The apical meristem of the shoot divides tubercle-like leaf systems on its edge, from which the leaves later emerge, which, in contrast to the shoot axis, have limited growth. In her armpits, however, an initially dormant meristem (armpit bud) remains, from which a side shoot can later emerge. The root branches out in a completely different way: lateral roots have their origin in the interior of the main root, where the pericycle, the outermost cell layer of the central cylinder , as a temporarily dormant descendant of the apical meristem, becomes active again after some time and forms a new root tip, which forms the one further outside Breaks through cell layers.
lifespan
Annual plants complete their life cycle from germination to seed maturation within a year. Their meristems are depleted, and after the seeds mature, the plant dies. Biennial plants do not bloom until the second year and then die; with perennial plants this happens after more than two years. On the other hand, perennial plants can live for many years and bloom and produce fruit every year. This includes all trees and shrubs, but also some grasses such as the North American buffalo grass Buchloe dactyloides , which spreads in a turf shape and of which a 10,000 year old specimen was found. Many other grasses, including all cereals, are annual.
Cellular Aspects
At the cellular level, too, the growth of plants differs considerably from that of animals. While animal cells divide in that a contractile ring is formed peripherally in the division plane, which like muscle fibers consists of actin and myosin filaments and divides the cell through centripetal constriction, the division of plant cells begins in the center of the division plane and proceeds centrifugally by a new cell wall is embedded. Here, too, the plane of division is determined by a ring-shaped structure in the periphery, the preprophase band consisting of microtubules . However, this disappears at the beginning of the nucleus division and is not involved in the subsequent cell division. A fundamental difference to the situation in animals is that the increase in cell volume (elongation growth) occurs entirely or 90% through the accumulation of water in the vacuoles, while in animal cells it results solely from an increase in the cytoplasm . In plants, the direction of cell extension, like the alignment of the cell division plane, is determined by microtubules in the cell periphery: in stretching areas of the cell, the microtubules are oriented perpendicular to the direction of extension, and on the other side of the cell membrane, cellulose microfibrils become parallel to the Microtubules are newly formed, which dictates the direction of elongation because cellulose is not stretchable.
swell
- Joachim W. Kadereit, Christian Körner, Benedikt Kost, Uwe Sonnewald: Strasburger Textbook of Plant Sciences . Springer Spectrum, Berlin / Heidelberg 2014, p. 263f.
- Jane Reece & al .: Campbell Biology . 10th ed., Pearson, Hallbergmoos 2016, pp. 1002-1016.