Structure And Physiology Of Angiosperms

Figure 7.2 Microscopic cross sections through an apical meristem. (From Weier et al., 1970.)

place where growth occurs in the stem. Xylem is formed toward the inside of the cambium, and phloem toward the outside. Each season produces a new layer of xylem, visible as the ''tree rings'' of a stump. Wood consists of dead xylem cells, most of which still conducts water and minerals. A tree can be killed without cutting it down simply by destroying the cambium all the way around the tree, a practice known as girdling. Lengthening of a plant occurs mostly by growth at the tip of the shoot, called the terminal bud or the apical meristem (Figure 7.2). Other buds produce branches or leaves.

The root is the underground portion of the sporophyte. Their main function is anchorage and absorption, but they may also be used in storage (as in carrots and potatoes). Monocots form a shallow fibrous root system. Gymnosperms and most dicots form a main root called a taproot that grows straight down. Roots of some trees have been found to penetrate 30 to 50 m into the soil. Most of the tree roots involved in absorption are in the top 15 cm and extend out beyond the crown of the tree. Roots of the corn plant (Zea mays) penetrate up to 1.5 m, and spread horizontally 1 m around the plant. The surface area of roots is greatly increased by the formation of root hairs, which grow from cells at the surface of the roots (Figure 7.3). As the plant grows, it maintains a balance between the leaf surface area and root surface area, so that water, minerals, and carbohydrates are formed in the proper proportion for growth. The roots absorb minerals by active

Figure 7.3 Microscopic view of root with root hairs. (From Simpson and Orgazaly, 1995.)

transport, which requires energy. Water transport, on the other hand, is mostly passive, powered by evaporation in the leaves.

The leaf is the main photosynthetic organ of the plant. Its large surface area is designed to capture sunlight. A cross section through the leaf (Figure 7.4) shows that the upper and lower surfaces of the leaf are formed of a layer of cells called the epidermis, which is covered by a waxy cuticle to minimize water loss. The cells between these two layers form the mesophyll, which has two parts. The palisade parenchyma form vertical columns under the upper surface. They are the main photosynthetic tissues of the leaf. The rest of the mesophyll consists of spongy parenchyma, which has a about 15 to 40% void volume filled with air and a large surface area for exchange of carbon dioxide, oxygen, and water vapor.

The epidermis is punctuated by special pores called stomata (singular, stoma or stomate). There can be as many as 12,000 stomata per square centimeter, mostly on the lower surface of the leaf, where they occupy about 1% of the leaf surface. Each stoma is made up of a pair of banana-shaped guard cells. The guard cells have chloro-plasts, whereas the other epidermal cells do not. The stomata control the passage of gases into and out of the leaf, thus affecting photosynthesis and water use. They also affect the transport of pollutants into the leaf. When the guard cells accumulate water, they swell, causing them to bow outward, opening the pore. Conversely, loss of water causes the pore to close. This occurs in response to several conditions. First, and most obviously, if water is scarce, a general wilting will cause the stomates to close, limiting water loss. This gives the plant control over transpiration, the evaporation of water from the plant. This "loss" is actually necessary for the plant's survival, giving it the means to transport minerals

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