Q.Read the following sentences and identify the correctly matched sentences.
i. In exarch condition, the protoxylem lies outside of metaxylem.
ii. In endarch condition, the protoxylem lie towords the centre.
iii. In centarch condition, metaxylem lies in the middle of the protoxylem.
iv. In mesarch condition, protoxylem lies in the middle of the metaxylem.
a. i, ii and iii only
b. ii, iii and iv only
c. i, ii and iv only
d. All of these

Xylem (Gk. xylos = wood; term coined by Nageli, 1858; older synonym hadrome, Haberlandt 1914) is the complex, principal water- and mineral-conducting tissue of a vascular plant, built from four cell types working together: tracheids, vessels (trachea), xylem parenchyma and xylem fibres. Xylem laid down by the procambium of the apical meristem is primary xylem; xylem added later by the lateral vascular cambium is secondary xylem. Within primary xylem, the earliest-formed, smaller-celled elements are protoxylem, and the later-formed, larger-celled elements are metaxylem; their relative spatial arrangement is named exarch (protoxylem peripheral, metaxylem central - typical of roots), endarch (protoxylem central, metaxylem peripheral - typical of stems), centrarch (protoxylem central, one strand, surrounded by metaxylem, e.g. Selaginella) or mesarch (protoxylem central, several strands, surrounded by metaxylem, e.g. Ophioglossum). Tracheids are dead, elongated, tapering, imperforate cells with bordered pits, showing annular/spiral/reticulate/scalariform/pitted secondary-wall thickening; they are the sole water-conducting cells of gymnosperms and pteridophytes and also give mechanical support. Vessels (trachea) are wider, dead, tube-like structures formed from vessel elements joined end to end and perforated at their end walls (a single pore = simple perforation plate, e.g. Mangifera; several pores = multiple perforation plate, e.g. Liriodendron); they are the chief water-conducting elements of angiosperms, essentially absent from pteridophytes and most gymnosperms (though present in a few, e.g. Gnetum, Ephedra, Welwitschia), and absent even in a handful of vesselless angiosperm families (Winteraceae, Tetracentraceae, Trochodendraceae). Xylem parenchyma is the only living tissue in xylem, storing food and assisting conduction (axial parenchyma runs along the organ's long axis; ray parenchyma runs radially). Xylem fibres (also called libriform fibres) are dead, lignified, mechanical cells, distinct from the transitional fibre-tracheids that sit structurally between typical fibres and typical tracheids.