CHAPTER 1
An introduction to cells and their organelles
William V. Dashek
Retired Faculty, Adult Degree Program, Mary Baldwin College, Staunton, VA, USA
Cells
Parenchyma, chlorenchyma, collenchyma, and sclerenchyma are the four main plant cell types (Figure 1.1, Evert, 2006). Meristematic cells, which occur in shoot and root meristems, are parenchyma cells. Chlorenchyma cells contain chloroplasts and lack the cell wall thickening layers of collenchyma and sclerenchyma. Certain epidermal cells can be specialized as stomata that are important in gas exchange (Bergmann and Sack, 2007). The diverse cell types (Zhang et al., 2001; Yang and Liu, 2007) are shown in Table 1.1. Photomicrographs of certain of these cell types can be found in Evert (2006), Fahn (1990), Beck (2005), Rudall (2007), Gunning (2009), MacAdam (2009), Wayne (2009), Beck (2009), Assmann and Liu (2014) and Noguchi et al. (2014).
Figure 1.1 Plant cell types: Left: parenchyma (par) and collenchyma (co). Right: sclerenchyma.
Source: Evert (2006). Reproduced with permission of John Wiley & Sons.
Table 1.1 Plant cell types.
| Cell types | Characteristics | References |
| Epidermal cells | Unspecialized cells; one layer of cells in thickness; outer covering of various plant parts; variable in shape but often tabular | Evert (2006) |
| Guard cells | Specialized epidermal cells; crescent shaped; contain chloroplasts; form defines stomatal pore | Wille and Lucas (1984) |
| Trichomes | An outgrowth of an epidermal cell; can be unicellular or multicellular | Callow (2000) |
| Parenchyma cells | Isodiametric, thin‐walled primary cell wall; in some instances may have secondary walls; not highly differentiated; function in photosynthesis, secretion, organic nutrient and water storage; regeneration in wound healing | Evert (2006) and Sajeva and Mauseth (1991) |
| Transfer cells | Specialized parenchyma cells; plasmalemma greatly expanded; irregular extensions of cell wall into protoplasm; transfer dissolved substances between adjacent cell; occur in pith and cortex of stems and roots; photosynthetic tissues of leaves; flesh of succulent fruits; endosperm of seeds | Dashek et al. (1971) and Offler et al. (2003) |
| Collenchyma cells | Lamellar or plate collenchyma, with thickenings on the tangential walls |
| Vascular cells | | Evert (2006) |
| Companion cells | Specialized parenchyma cells; possess numerous plasmodesmatal connections | Oparka and Turgeon (1999) |
| Albuminous cells in gymnosperms | Absence of starch; cytoplasmic bridges with sieve cells; dense protoplasm, abundance of polysomes, highly condensed euchromatin and abundant mitochondria | Alosi and Alfieri (1972) and Sauter et al. (1976) |
| Tracheids Vessels | Long tapering cell with lignified secondary wall thickenings; can have pits in walls; devoid of protoplasm at maturity; not as specialized as vessels; widespread | Tyree and Zimmerman (2002) Fukuda (2004) and Evert (2006) |
| Specialized cells – Hydathodes (modified parts of leaves and leaf tips or margins) | Consist of terminal tracheids epithem, thin‐walled chloroplast‐deficient cells, a sheath with water pores; guttation discharge of liquid containing various dissolved solutes from a leaf’s interior | Lersten and Curtis (1996), https://www.biosci.utexas.edu/ and Maeda and Maeda (1988) |
| Laticifer cells | Cells or a series of cells which produce latex | Fahn (1990), Pickard (2008) and Botweb.uwsp.Edu |
| Compound and articulated | Union of cells compound in origin and consist of longitudinal chains of cells; wall separating cells remain intact, can become perforated or entirely removed | |
| Salt glands | Modified trichomes, two‐celled and positioned flat on the surface in rows parallel to the leaf surface; occur in Poaceae; | Evert (2006), Tan et al. (2010), Oross et al. (1985) and Thomson et al. (1988) |
| Cap cell – large nucleus and expanded cuticle | Naidoo and Naidoo (1998) |
| Basal cell – numerous and large extensive partitioning invaginations of plasmalemma | |
| Nectaries | Found in nectarines; produce nectar, usually at the base of a flower | Fahn (1990), Nicolson and Nepi (2005) and Paiva (2009) |
| Idioblasts | Crystal‐containing cells | Lersten and Horner (2005) |
| Raphides | Produce needle‐shaped crystals | |
| Mucilage cell | Occur in a large number of dicots, common in certain cacti; slimy mucilage prevents evaporation of water by binding to water; a parenchyma cell whose dictyosomes produce mucilage as in seed coats; cell walls are cellulosic and unlignified | http://www.sbs.utexas.edu/masuetl/weblab/webchap9secretory/9.1‐2.html, Western et al. (2000) and Arsovskia et al. (2010) |
| Oil cells | Specialized cells appear like large parenchyma cells; can occur in vascular and ground tissues of stem, and leaf cell wall has three distinct layers; cavity is formed after the inner wall layer has been deposited | Rodelas et al. (2008), http:brittanica.com and Lersten et al. (2006) |
| Druses | Spherical aggregates of prismatic crystals | Lersten and Horner (2005) |
| Leptoids – Pteridophytes | Organic compound‐conducting cells; sporogenous cells present in sporangia of sori | |
How do cells arise?
Cells arise by cell divisions (see Chapter 8 for mitosis and meiosis) in shoot and root (Figures 1.2 and 1.3) meristems (Table 1.2, Lyndon, 1998; McManus and Veit, 2001; Murray, 2012). The shoot apex is characterized by a tunica–corpus organization (Steeves and Sussex, 1989). The tunica gives rise to the protoderm and its derivative, the epidermis. In contrast, the corpus provides the procambium which yields the primary xylem and phloem. In addition, the ground tissue derives from the corpus originating the pith and cortex. Following divisions, cells can differentiate into tissues (Table 1.3) and organs of the mature plant body (Leyser and Day, 2003; Sachs, 2005; Dashek and Harrison, 2006). The leaf primodium arises on the apex (Micol and Hake, 2003). The mature angiosperm leaf consists of palisade cells and spongy mesophyll cells sandwiched between the upper and the lower epidermis (Figure 1.4). The epidermis possesses guard cells with associated stomata that function in gas exchange. KNOX genes affect meristem maintenance and suitable patterning of organ formation (Hake et al., 2004). In dissected leaves, KNOX genes are expressed in leaf primordia (Hake et al., 2004). Hake et al. (2004) suggest that KNOX genes may be important in the diversity of leaf form. Extensive discussions of leaf development occur in Sinha (1999), Micol and Hake (2003) and Efroni et al. (2010). Under appropriate stimuli the vegetative apex can be converted to a floral apex (Figure 1.5). Photoperiod (Mazumdar, 2013), such as short days and long days and combinations of the two, is one such stimulus (Glover, 2007; Kinmonth‐Schultz et al., 2013). This induction results in the production of florigen (Turck et al., 2008), the flowering hormone (Zeevaart, 2006). While early reports suggest that florigen is...
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