ROOT PRESSURE
If the stem of a young seedling is cut off just above the soil, the stump will often exude sap from the cut xylem for many hours. If a manometer is sealed over the stump, positive pressures can be measured. These pressures can be as high as 0.05 to 0.5 MPa. Roots generate positive hydrostatic […]
Diffusion
The net movement of molecules from region of high concentration to regions of low concentration that is, down the concentration gradient. Water molecules in a solution are not static they are in continuous motion, colliding with one another and exchanging kinetic energy. The molecules intermingle as a result of their random thermal motion, this random […]
WATER IN THE SOIL
The water content and the rate of water movement in soils depend to a large extent on soil type and soil structure. At one extreme is sand, in which the soil particles may be 1 mm or more in diameter. Sandy soils have a relatively low surface area per gram of soil and have large […]
WATER IN PLANT LIFE
Water makes up most of the mass of plant cells. Each cell contains a large water-filled vacuole. In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole. Water typically constitutes 80 to 95% of the mass of growing plant tissues. Common vegetables such as carrots and […]
Sources and Sinks
Sources include any exporting organs, typically mature leaves, that are capable of producing photosynthate in excess of their own needs. The term photosynthate refers to products of photosynthesis. Another type of source is a storage organ during the exporting phase of its development. For example, the storage root of the biennial wild beet (Beta maritima) […]
PATHWAYS OF TRANSLOCATION
The two long-distance transport pathways—the phloem and the xylem—extend throughout the plant body. The phloem is generally found on the outer side of both primary and secondary vascular tissues. In plants with secondary growth the phloem constitutes the inner bark. The cells of the phloem that conduct sugars and other organic materials throughout the plant are […]