B

Figure 1.9. Plants are composed of several tissues.

are branched cells that are connected electrically by gap junctions (page 55), and their automatic rhythmical contraction powers the beating of the heart. Each skeletal muscle is a bundle of hundreds to thousands of fibers, each fiber being a giant single cell with many nuclei. This rather unusual situation is the result of an event that occurs in the embryo when the cells that give rise to the fibers fuse together, pooling their nuclei in a common cytoplasm. The mechanism of contraction of skeletal muscle will be described in Chapter 18.

Plants

Plant cells are also organized into tissues (Fig. 1.9). The basic organization of a shoot or root is into an outer protective layer, or epidermis, a vascular tissue that provides support and transport, and a cortex that fills the space between the two. The epidermis consists of one or more layers of closely packed cells. Above the ground these cells secrete a waxy layer, the cuticle, which helps the plant retain water. The cuticle is perforated by pores called stomata that allow gas exchange between the air and the photosynthetic cells and also constitute the major route for water loss by a process called transpiration. Below ground, the epidermal cells give rise to root hairs that are important in the absorption of water and minerals. The vascular tissue is composed of xylem, which transports water and its dissolved solutes from the roots, and phloem, which conveys the products of photosynthesis, predominantly sugars, to their site of use or storage. The cortex consists primarily of parenchyma cells, unspecialized cells whose cell walls are usually thin and bendable. They are the major site of metabolic activity and photosynthesis in leaves and green shoots.

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