|Growth and Growth Control|
Plant growth is of two distinct types: primary growth and secondary growth. Primary growth results in increased length of stems or roots. Secondary growth increases the width of the plant and allows differentiation of cells into various distinct tissue types. Both types of growth occur in plant tissues called meristems.
A meristem consists of tissue where extensive cell division takes place, and thus plant growth. There are two general types of meristems. Primary growth occurs at the apical meristems, and secondary growth occurs at the lateral meristems, which are known as the vascular cambium and the cork cambium.
Apical meristems are located at the growing tips of the plant; there are apical meristems in the roots and in the buds on shoots of the above ground part of the plant. New cells produced at the meristems are initially undifferentiated.
Other meristematic tissue occurs along the stem, and lateral buds are capable of producing branches with their own meristems, but most elongation occurs from the apical meristem. Apical dominance prevents excessive branching and in some plants prevents all branching.
If the apical meristem is removed by removing the end of the stem, the lateral buds will be released from apical dominance, and greater branching results. Eventually, one of the lateral meristems will grow more than the others and will impose apical dominance, becoming the new apical meristem.
Cork cambium produces cells at the outer edges of roots and stems. At maturity these cells are dead and form the bark, their primary function being structural support and protection. The walls of cork cells contain a protective waxy substance called suberin.
Gametophytes and Sporophytes
All vascular plants, as well as mosses and liverworts and many algae, display a type of life cycle referred to as alternation of generations, which involves two distinct life-forms, the gametophyte and sporophyte.
The sporophyte generation is genetically diploid and, as the name implies, produces spores by meiosis. Spores germinate and develop into gametophytes, which are genetically haploid and produce gametes (eggs and sperm) by mitosis.
Sporophytes are larger than gametophytes and represent the dominant, or more noticeable, generation and typically live much longer than gametophytes. Trees, shrubs, herbs, and ferns all represent sporophytes. Gametophytes are extremely small by comparison and are therefore unknown to most non scientists.
|Gametophytes and Sporophytes|
In mosses and liverworts, gametophytes are larger and represent the dominant generation, and the sporophyte grows from the structure on the end. Algae are more diverse. In some groups both gametophytes and sporophytes are indistinguishable, whereas in other groups one or the other is larger.
Both the gametophyte and the sporophyte develop from single cells—the spores and the zygotes, respectively. In seed plants, the first cell division of the zygote often defines the root cell line (or its equivalent) and the stem cell line (or its equivalent).
The body produced by this early development is initially linear in many cases, laying out the primary axis of the plant body. The embryo grows from the zygote and as it matures is included in integuments that develop into the seed coat. A primary root and primary stem grow from a root apical meristem and a shoot apical meristem, respectively.
Primary and Secondary Tissue
In many species, the new cells in the sporophyte are produced primarily by the division of apical meristems, thus consisting almost exclusively of primary tissues. There are, however, some plants in which the sporophytes grow in girth.
Some of these—such as the calamites (giant horsetails), the Lepidodendron (tree lycopods), and the seed ferns are known only from the fossil record. Others are the trees and shrubs, so-called woody plants, that characterize the modern forests.
The wood of woody plants is composed almost entirely of secondary xylem—xylem that is not derived from the apical meristems but instead grows from the vascular cambium, a cylindrical meristem located under the bark.
The bark of woody plants, also a secondary tissue, is composed of phloem and corky layers. The corky layers develop from a second cylindrical meristem, the cork cambium.
In addition to the secondary tissues, many plants as they grow produce secondary organs: branch stems and branch roots. These secondary organs are not derived from the original axis of the plant.
In the early stages of their growth they are composed of primary tissues that are essentially identical to the primary tissues of primary organs. Cambial growth will produce secondary tissues in these branch stems and roots. The patterns of secondary tissue formation determine the form of the wood and bark of woody species.
The patterns of secondary organ formation determine the architecture of the plant: the shape of the crown and the root system. This architecture plays an important role in the ability of the plant to compete for sunlight, water, and soil nutrients.
In many cases, the dwarfing results from a failure of the stem to elongate in the internodes (the regions between the nodes, where leaves and lateral branches originate).
Dwarfing appears to be particularly influenced by plant hormones called gibberellins, which stimulate internodal elongation in dicots. The effect of gibberellins is also influenced by the concentration of the other hormones within the plant.
Likewise, the architecture of columnar plants is under genetic and hormonal control. The Lombardy poplar, for example, has greatly reduced branching compared to the European poplar. This elongation of the principal axis is similar to that found in forest trees growing in the shade of the surrounding forest.
The shaded environment stimulates the growth of the main axis of many tree species while inhibiting growth of the secondary stems. As a result, the stem reaches above the surrounding trees and is better able to compete for light.
The inhibition of secondary stem formation seems to be influenced primarily by hormones called auxins. High auxin concentrations inhibit the development of secondary stems, while low auxin levels stimulate the formation of branches. In some species, high cytokinin levels also stimulate secondary stem growth.
Because cytokinins are produced in large quantities in the root tips, and auxins are produced in large quantities in the stem tip, the relationship between these two chemicals reflects the balance between the root system and the stem system.
Less is known about the mechanisms of control of branching in the root system. Some species, especially monocots, have many secondary roots of approximately equal size. Others have dominant primary roots, called taproots, with little development of secondary roots. Carrots carry this pattern to an extreme.
The pattern of secondary tissue formation is determined by an interplay between genetic factors and environmental conditions. Many plants complete their life cycles in a single year. This quick passage from seed to seed is under genetic control. Annuals rarely develop woody tissue; perennials survive many seasons and show increases in stem girth throughout their lives.
The annual rings seen in the cross-section of a tree are a result of seasonal variations in the production of secondary xylem. Variations in the thickness of annual rings are the result of genetic controls and environmental factors such as mean temperature, damage from insect pests or other pathogens, and water and nutrient availability.
Even wind can have significant effects. Strong prevailing winds cause an effect called wind pruning, which results in reduced branching and shorter distances between the annual rings on the windward side.
Secondary tissues are extraordinarily complex. The patterns of cell division, apparently under genetic control, are influenced by a whole concert of hormones. Hormonal gradients and seasonal gradients of sugars and amino acids may play a role in these patterns of secondary tissue formation.
The activity of mature phloem tissues and the concentrations of auxins, gibberellins, cytokinins, and perhaps of the gaseous hormone ethylene may all be important in regulating the activity of the cambia.