When talking about the features of the tissues of plants, one has to keep in mind their evolutionary history. In the middle paleozoic period, about 450 to 500 million years ago, plants began to colonize the land, where the environment offered some advantages over the aquatic environment: more hours of light, more light intensity, and more free movement of CO2. In return, plants had to solve new challenges, most of them related to obtaining and retaining water, keep an erect body, as well as the dispersion of seeds in the air. To solve these problems, plants grouped specialized cells to form tissues with specific functions that were able to cope with these new challenges. In plants, tissues form tissue systems. According to topographical locations, the term tissue system (Sachs, 1875) is used to highlight the organization of tissues into larger entities. In a higher level, different tissue systems are grouped to form organs.
Plant tissues have been traditionally grouped in the following tissue systems: protection (epidermis and periderm), ground tissues (parenchyma, collenchyma, and sclerenchyma), and vascular (xylem and phloem). The protection system allows plants to survive in dry and variable environments. It is the outer part of the plant and, depending on the plant species and development state of the plant organ, is the epidermis or periderm. Cell of these tissues are covered by cutin and suberin, waxes that avoid or decrease the water loss. Furthermore, in the epidermis, plants develop stomata, which are organizations of specialized cells for regulating transpiration and gas interchange between the plant and the air. Epidermis also develops trichomes with several functions. The ground system is mainly involved in metabolic and mechanical support functions. Most of the ground system is parenchymatic tissue, which carries out functions like photosynthesis and storing different types of substances. Furthermore, the ground system contains cells specialized in mechanical support like collenchyma and sclerenchyma, which keep upright the body of many herbaceous plants and contribute to give the final shape of many plant organs. The vascular system is one of the most relevant novelties during plant evolution because it communicates the different organs and parts of the plant by transporting molecules and water. This system is composed of phloem, which transports organic substances diluted in water, and xylem, which transports mainly water and inorganic substances. Besides communication, vascular system became the main support tissue in larger plants like trees and bushes. Both, communication and mechanical support, led plants to increase in size and to colonize many nonaquatic territories. Only vascular plants contain true conducting tissues.
Plant tissues may be classified in other ways. For instance, attending to the cell diversity found in the different tissues, there are simple tissues with only one type of cell, like parenchyma, and complex tissues with several cell types, like protection and vascular tissues.
Finally, vascular plants produce seeds. Inside the seed, the embryo develops and grows thanks to the activity of the embryonic or meristematic tissues. Meristems are not only present in the embryo, but are active through the lifespan of the plant, thus allowing the permanent growth of the plant body.
Tissues and tissue systems are grouped together to form the plant organs that can be classified as vegetative, such as the root (organ responsible of water and salts uptake), the stem (organ for the transport of substances, to sustain the aerial plant body, and sometimes also to carry out photosynthesis) and the leaf (organ that captures solar energy and performs photosynthesis, and it is responsible for the gas and water interchange regulation of the plant). Others are classified as reproductive organs, such as the flower and its derivatives, the seed and the fruit. Tissue systems are distributed in characteristic patterns depending on the organ.
Before going into the study of each of the tissues and organs, it is necessary to understand two points that are characteristic of plants:
1.- Plant cells have an outer structure, covering their plasma membrane, synthesized and released by the cell. This structure is the cell wall. It provides mechanical support in absence of a well-developed cytoskeleton and determines the size and shape of the cells, the texture of the tissue, and the form of the organs. Different cell types are identified by the morphological structure and chemical compositon of their cell walls. The cell wall originates during cell division. During cytokinesis, pectic substances are deposited between the two daughter cells forming a cell wall layer called middle lamella. Pectic substances are adhesion molecules that tend to keep cells together. Afterward, each cell synthesizes its own primary cell wall layer on each side of the middle layer. Primary cell wall is mainly made of cellulose and hemicellulose. The primary wall is deposited while the cell is growing. Some cells can also synthesize a secondary layer that, in addition to cellulose, usually contains lignin. This secondary wall is characteristic of some specialized cells and is mostly deposited when the primary wall stops growing. Plant cells contain middle lamella and a more or less thick primary cell wall, but only a few cell types have secondary cell wall.
2.- Plants develop and grow from embryo to adult thanks to the activity of meristems. The initial growth of plants, and the only one in some groups, is the growth in length. This is referred as primary growth, wich is produced by the activity of a group of meristematic cells that are found at the tip of the stems and roots, as well as at the base of the internodes. These groups of cells are the primary meristems. In addition, some groups of plants can also grow in thickness, a type of growth known as secondary growth, which depends on the activity of another type of meristems known as secondary meristems.
Sachs J. 1875. Text-book of botany: morpholoical and physiological. Clarendon Press. London.