trachea are elongated cells, with thick walls and lose their cytoplasm at maturity, so k is dead cells. The work. Terminal walls dissolve and form continuous tubes called vessels. sieve tube: as k the elements of glass, hese cells are at the end to end and form long sieve tubes are also .------- polyhedral cells of epithelium lining the trakea k in this case its cells have cilia beat synchronously k to move substances. DNSO: the fibers are arranged in different directions and at different levels, it allows them to endure k stretching in all directions. Internal environment: the unicellular conduct trade directly with the extracellular environment in the k live, but in most multicellular cells are not in direct contact cn the external environment. The organ. Multi. Indispensable the existence of a liquid medium. Call us internal environment, functioning as an intermediary between the external environment and the intracellular environment. Benefits: - cells provides a suitable environment for their operation. The internal milieu k provides the nutrients the cells need and receive their waste. - Allows diferents exchange between cells. Each cell, or each group of cells, it provides some k all the other needed and the internal environment allows k to be distributed and available to those who need them. - Provides a relative independence from the body with respect to k changes occurring in the environment. Maintains the internal environment, more or less constant, its char., k the proportion of cells k stable conditions make operation easier. in the plant, the internal environment consists of k likidos circulate through the spaces Intercellular x inside the xylem vessels and phloem. in animals the internal environment can be subdivided into blood plasma and interstitial likidos. Plasma is inside the blood vessels and lymph, and interstitial LIKIDO occupies the space between cells. homeostasis: is the set of physiological processes charac k stable. The internal environment. in animals that this stability is maintained by coordinated activity of the circulatory, nervous and hormonal systems and, in particular organ k Exchanges with the external environment such as the kidneys, skin, lungs or gastrointestinal tract.