Sieve elements are thin-walled cells that are alive at maturity, although the protoplast is greatly changed, and they generally lack nuclei. Sieve elements are elongated and function as the basic photosynthate-conducting cell type in the phloem of vascular plants. The walls of sieve elements contain sieve areas, circular-to-elliptical parts of the wall that are thinner. Each sieve area (FIG. 7.11) includes a number of sieve pores, which allow for transport from one sieve element to the next (FIG. 7.12). Sieve pores are not actual holes in the wall as perforation plates are. Rather, they are protoplasmic connections between two living cells and are lined with a plasma membrane. In addition to the basic sieve element, there are two more specialized types of sieve elements: sieve cells, which occur in conifers, and sieve tube elements, which are a synapomorphy for the angiosperms. Sieve cells are generally long, narrow, and tapered at the ends, whereas sieve tube members are shorter and wider with more horizontal end walls. Sieve tube elements, like vessel elements, are connected end to end in vertical rows to form sieve tubes. Sieve plates occur on the end walls of sieve tube elements; these are groups of sieve areas, usually with larger pores than those on the lateral walls of the cell. Since sieve elements are under great hydrostatic pressure while functioning, they often collapse after death. Thus, preservation of phloem tissue in fossils is relatively rare.
chapter 7 INTRODUCTION TO VASCULAR PLANT MORPHOLOGY AND ANATOMY
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