Trees are much taller than herbaceous or shrubby plants[citation needed]and ensuring the upper-most leaves are supplied with water originating in the root system requires that water is drawn up through the xylem from the roots by the suction produced as it evaporates from the leaves.[clarification needed] If insufficient water is available the leaves will die.[61] The leaves of trees come in a wide range of shapes and sizes which have evolved in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees. They are adapted for life in environments where resources are low or water is scarce. Frozen ground may limit water availability and conifers are often found in colder places at higher altitudes and higher latitudes than broad leaved trees. In many cases, their branches hang down at an angle to the trunk which decreases the likelihood of them breaking when weighed down by snow.[clarification needed] Broad leaved trees in temperate regions have a different[clarification needed] strategy for dealing with winter weather. When the days get shorter and the temperature begins to decrease, the leaves no longer makes new chlorophyll and the red and yellow pigments already present in the blades become apparent.[62] Synthesis in the leaf of a plant hormone called auxin also ceases. This causes the cells at the junction of the petiole and the twig to weaken and sooner or later the joint

reaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but most leaves remain intact for some time. Other tropical species and those in arid regions may shed all their leaves annually at a particular time of year.[citation needed] Often this will coincide with the onset of the dry season or some other climatic event.[clarification needed] Many deciduous trees flower before the new leaves emerge. Plant hormones (also known as phytohormones) are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved to other locations, in other locations of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also called as growth factors or growth hormones