Zea mays

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These vascular tissues occur nearer to the apex in slow growing or dormant roots than in actively elongaring roots. The firsr mature protOphloem elements appear at the circumference of the procambial cylindet and differentiate closer to the root apex than the first mature protOxylem (4.8). By contrast the prospective met'axylem becomes demarcated, by its prominent vacuolation growth (4.5), nearer the apex than the protophloem. Some distance basal ro the prorophloem, mature protoxylem elements develop between the prorophloem files. Maturation of the metaphloem and metaxylem proceeds centripetally (4.30) on the radii already demarcated by the protophloem and protoxylem (4.8), and the vascular system of the mature primary rOOt is formed (4.31). The outer layer of the procambium gives rise 10 the parenchymatous pcricyle (4.8,4.31). In dicotyledonous roots the tissue between the xylem and phloem often forms a cambium (4.31), which later spreads laterally over the protoxylem poles ro form a COntinuous meristem (1.25). The vascular cylinder is invested by the parenchymatous cortex (1.29, 4.30} which frequently contains conspicuous intercellular air spaces. The pericycle is bounded by the single layered endodermis (1.29, 4.31, 4.32) whose radial and transverse walls are impregnared with lignosuberin to form Casparian bands (4.33). These laterally continuous bands are impermeable, so that all water and solute movement across Ihis layer is confined to the symplast (4.33). In the roots of many species, particularly monocotyledons, the ligno-suberisation of the endodermis 86 later extends to all walls and additional cellulose thickening may be deposited (4.32). Such cells, however, still allow symplastic transport from rhe cortex ro the stele via their plasmodesmata. The endodermis is an important selective barrier but it allows the active transport into the vascular cylinder of certain beneficial ions (potassium, phosphare) absorbed from the soil. Calcium, however, apparently moves apoplastially and cannor corer the vascular cylinder through the endodermis, but instead passes into the vascular cylinder in the very immarure root where Casparian bands have not developed. The active transport of ions into the vascular cylinder accounts for the phenomenon of roOt pressute which sometimes plays an accessory role to transpiration in the movement of water to Ihe shoot. to some roots the ourer cortex diffetentiates as a one- ro several-layered exodermis (1.29) but the Casparian bands in the tadial walls are usually masked by deposition of subetised lamellae adjacent ro the protoplast (4.34). A short distance behind the apex a zone of absorptive root hairs develops from the epidetmis (4.27); but warer absorption also occurs over the test of the epidermal surface and in some species roar hairs are absent. In the aerial roots of some epiphytic orchids and aroids the multilayered epidermis develops into a dead velamen (4.34), whose walls are thickened by bands of cellulose. This is probably an adaption to absorb watet from the humid atmosphere of tropical forests. 4.1 lu.s of the plwnule of Pht'lseo/us /lUfgaris (bean) seed. The hemispherical, densely-staining and small-eelled shoot apex (1) bears a pair of leaf primordia (2) at its margins which represent the first foliage leaves. A prominent strand of procambium (3) demarcates tbe longitudinal rows of vacuolated pith (4) from the cortex. (LM x 110.) 1 Shoot apex 2 Leaf primordium 3 Procambium 4 Pith
4.2 4.3 r-----;; 4.2 RLS of the extreme lip of the vegetative shoot apex of the dicol),ledon Gfechoma hederacea (ground ivy). )olore: the single: tunica layer (1) where onl}' anticlinal divisions occur and the undc:r1}'ing corpus (2) which divides in various planes. The small thin-walled cells possess large nuclei (3) while the cytoplasm is relarivdy unvacuolared (d., 4.3). (KMn, TIM x 1,830.) 1 Tunica layer 2 Corpus 43 RLS of the \"egcGlriH: shoot ape."< of the: dicoryledon Glechoma hederacea (gr-ound ivy). Notc: the pair of leaf primordia (1) .at itS base: and a pair of bud primCH"dia (2) in their axils. The txtec:mc: tip of the: apex (d., the corresponding area in 4.2) consists of small densely c:ylOplasmil; cells, bUl the rib mc:ristc:m cells (3) below are vacuolating and become: the: pirh (4) of the: roung 5u:m. The: margins of the: apex consi!r of densely-staining flank meristem cells (5). l'rocambium (6). (G-Os, L\1 x 285.) 1 1 Procambium 2 Flank mcnstcm 1 Leaf primordium 2 Bud primordium 3 Rib meristc:m 4 Pith 5 ..lank mc:ristc:m 6 Proc.amhium 3 :-Jucleus 4.4 4.4 LS of rhe vegerarive shoO! apex of rhe dicoryledon C/ewoma hederaua (ground h7). This section is slightly tangential to the same apex shown previously (d., 4.3). Note how the narrow, densely cytoplasmic procambial cells (I) merge with the flank merisrem (2) within the shoot apex. Cortex (3). (G-Os, 1.M x 285.) 3 Cortex 87
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A Colour Atlas of PLA T STRUCTURE B
Page 4:
5 The green leaf 97 Introduction 97
Page 7 and 8:
• CHAPTER 1 Introduction The asso
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1.6 1.6 L}'copusicon e$culentum (to
Page 15 and 16:
1.17 1.19 ,--------------::c-=-----
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1.22 1.22 Group of rail Cordy/hIe a
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1.32 B 24 1 6 \ -H-r 14 13 c 11 """
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times becomes highly lobed (2.15, 2
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stack (2.14, 2.52) of smooth cister
Page 34: 2.14 2.14 Diagram of the fine-struc
Page 43: 2.39 Longitudinally-sl:crionl:d l:h
Page 55: CHAPTER 3 Plant histology Disttibut
Page 60: 3.1 3.2 3.1 TS of the: petrified st
Page 66: 3.21 ,-------------, 3.23 3.2J Non-
Page 69: 3.31 TS of n cortienl mucil:lge duc
Page 72: 3.41 -+-2 3.39 L$ of the phloem of
Page 78: 3.58 80 3.56 3.56 TS of tbe seconda
Page 83: absent or may develop some distance
Page 91: 4.24 4.24 TS of a vascular bundle f
Page 97 and 98: secretion may temporarily accumulat
Page 99: 5.3 ,--------------, 5.3 Large fan-
Page 112 and 113: 5.45 5.44 IS of the petiole of the
Page 114 and 115: 5.50 116 5.50 Base of the mature tr
Page 116 and 117: fibres often develop in the outer p
Page 120: 6.7 6.7 5hool of the monocotyledon
Page 129 and 130: 6.33 6.32 TS showing distribution o
Page 134:
7.4 I'neumatophores of Alliceimia n
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7.14 l.S of a later:J.1 root primor
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7.26 7.26 Fine rOOfS from a large t
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pappus (calyx, 8.9) of each floret
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tube frequently corers the embryo s
Page 155 and 156:
S.19 TS of imm,lIure pollen sacs in
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8.25 8.25 Mature pollen grain of th
Page 169:
8.64 hnmarure fruir of rhe dleot)·
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a tetraploid h3.S a double: set. Po
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Vacuole An organelle bounded by rhe
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Cell (continued) meiosis 158, 161,
Page 188:
Sanicula europea (sanicle) 68 Sanse
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Zea mays

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