TISSUES WORKING TOGETHER 4.4 The primary function of the leaves of most plants is to perform photosynthesis. In order for photosynthesis to occur, plants need energy in the form of sunlight carbon dioxide water Photosynthesis uses the above items to produce glucose and oxygen. Glucose may then be converted into carbohydrates, including complex sugars. The carbohydrates, in the form of starch, are used as a source of stored chemical energy. Sugar is needed by all parts of a plant. Plants must transport sugar and oxygen from their leaves to the other plant parts for cellular respiration. Leaves produce more oxygen than a plant can use, so the excess is released into the air.

TISSUES WORKING TOGETHER 4.4 Most leaves are green and thin, making them ideal for absorbing light. Wide, thin leaves have a large surface area for absorbing light. The green colour is produced by chlorophyll, which absorbs light to begin the photosynthesis process. Chlorophyll is contained in cell organelles called chloroplasts. Chloroplasts conduct the chemical process of photosynthesis. Chloroplasts are located mostly in the palisade layer and the spongy mesophyll. Review the diagram to see where the cells are located. Notice that some of the cells are tightly packed and others are not. Why do you think these structures have developed differently? palisade layer spongy mesophyll

TISSUES WORKING TOGETHER 4.4 The leaf epidermis contains many tiny openings called stomata. (stomate = one) The stomata allow gas exchange and the release of water vapour. Stomata are usually located on the lower surface of a leaf, which reduces water loss provides more surface area for photosynthesis reduces the chance of airborne viruses, bacteria, and fungal spores entering the leaf. When stomata are open, carbon dioxide enters and oxygen leaves the leaf. Water vapour also escapes when stomata are open. stomata cuticle guard cells

TISSUES WORKING TOGETHER 4.4 The cuticle is a layer of wax on the upper and lower surface of a leaf. It prevents the release of too much water vapour, which would cause the leaf to dry out. The cuticle also prevents gases from entering the leaf by diffusion through the surface cells. Each stomate is surrounded by a pair of special epidermal guard cells that control its opening and closing. Guard cells change their shape to respond to water levels in a plant. They also close at night when carbon dioxide is not needed for photosynthesis. stomata cuticle guard cells

TISSUES WORKING TOGETHER 4.4 Long, thin epidermal cells on the roots of a plant (called root hairs) absorb water from the soil into the plant by a process called osmosis. Water is then transported by the xylem (vascular tissue) from the roots, up the stem, to the leaves. Although plants and animals are very different organisms, they have similarities at the cellular level. Plant and animal cells need to perform some of the same processes, such as respiration. In fact, both plant and animal cells use sugar and oxygen in the process of respiration. Animals and plants both possess systems that must work together to accomplish complex tasks.

PLANT GROWTH 4.6 Cell division occurs only in certain parts of a plant. Most differentiated plant cells cannot divide further. However, plants will continue to grow for as long as they live. Apical meristems are undifferentiated cells located at the tips of plant roots and shoots. These cells divide and enable the plant to grow longer and develop specialized tissue. The diagram shows three distinct regions of the growing tips of roots on a plant. apical meristems

PLANT GROWTH 4.6 When meristem cells divide, they become elongated. This makes roots longer and enables them to push their way through the soil. Cells in root tips may grow 10 times longer than their original size. In the region of maturation, elongated cells differentiate into specialized cells of the dermal, ground, and vascular tissue systems. After this process is complete, most cells can no longer grow or divide. Apical meristems also occur in buds at the very tip of growing stems. They also occur along stems, giving plants the ability to grow side branches off main stems.

PLANT GROWTH 4.6 Lateral meristems are undifferentiated cells located under the bark in the stems and roots of woody plants, such as trees. These cells divide and enable the plant to grow wider and develop specialized tissue in the stem. Lateral meristems form two cylinders—one inside the other—that run the full length of roots and shoots. As the tree grows in diameter, the outer lateral meristem produces new dermal tissue called cork. The cork replaces the old epidermal cells. The inner lateral meristem produces new phloem tissue on its outer surface and new xylem tissue toward its centre. The phloem and cork form the bark of the tree. The rings of xylem form the interior of the tree trunk. The accumulation of xylem produces a ring, which is used to determine the age of a tree. lateral meristems

Heart wood Heartwood ... in older trees, as new growth rings of sapwood are formed from the outside, inner rings of older sapwood, blocked with resins, become heartwood. Heartwood can no longer carry fluids, but its stiffness helps support the tree at the centre. Heartwood is generally darker than sapwood.

PLANT GROWTH 4.6 Clones are individuals that are genetically identical to each other. Many plants produce clones of themselves. This is called vegetative reproduction. Strawberry plants produce clones when they send out shoots called runners across the surface of the soil. The natural cloning ability of plants has been used for years by farmers and gardeners to produce new plants. Agricultural scientists use a process called tissue culture propagation to grow identical plant offspring. The scientists obtain individual plant cells from one parent plant and then grow them into calluses (clumps of cells), and finally into whole plants. Reproduced plants are genetically identical to their parent. vegetative reproduction tissue culture propagation