Mature (Permanent) tissues 68 C- Vascular system 68 - Vascular tissues are present in Pterophyta (ferns) and in Spermatophyta (seeded plants); both divisions are, some- times, termed as Tracheophyta (plants with vascular sys- tem formed of tracheae or simply vascular plants). - Vascular tissues include xylem and phloem tissues. - Xylem is the water conducting tissue; in addition it acts as a mechanical support to the different plant organs. - Phloem is the principal food-conducting tissue of vascular plants. - Both vascular tissues form the stele (or vascular column), they are found (with or without cambium) in vascular bun- dles (stems) or as separate radial strands (roots),surroun- ded by the endodermis & pericycle & surrounding the pith.

Mature (Permanent) tissues 69 C- Vascular system 69 - Vascular tissues of the primary body are derived from the procambial strands of the primary meristem situated below the growing apecies of the shoot or the root or associated with leaf primordium. - We will study : (1) The elements constituting the vascular tissues (i.e. Xylem & Phloem tissues); (2) Types of vascular bundles; (3) Types of stele (or the stelar structure).

Mature (Permanent) tissues 105 C- Vascular system 105 The Stele - Xylem and phloem are found in close association in the plant body, and together form what is termed the stele which also includes the pericycle to the outside and the pith centrally. Types of steles: I- Protostele: - In which the vascular tissue is a single mass in the centre of the plant organ (i.e. there is no pith) . - There are 3 different types of protostele: a- Haplostele: is the simplest type in which the xylem appears ± circular in cross section, surrounded by the phloem and there is no pith (e.g. Rhynia « fossil »);

Mature (Permanent) tissues 106 C- Vascular system 106 b- Actinostele: in which the xylem may become star shaped (Stellate), and there is no pith, e.g. Pisalotum. c- Plectostele: is more advanced than actinostele, where the xylem is divided into separate plates of which some are joined and others are, usually, separated by phloem, and there is no pith, e.g. Lycopodium clavatum.

Mature (Permanent) tissues 107 C- Vascular system 107 II- Siphonostele: - stele of this type has a central column of pith. - Various types of siphonosteles are distinguished according to the position of xylem and phloem as follows: (i) Ectophloic sipholostele: the pith is surr- ounded by the xylem which in turn is surr- ounded by the phloem so the phloem is present on the outer side of the xylem only ( e.g. Nicotiana & Salix ). (ii) Amphiphloic siphonostele: the pith is surrounded by inner endodermis, inner

Mature (Permanent) tissues 108 C- Vascular system 108 pericycle, inner phloem, xylem, outer phloem, outer pericycle, and outer endodermis. (common in ferns, e.g. Adiantum, Marsilea,..). III- Solenostele : - This type of stele originates from either ectophloic or amphiphloic siphonostele, by the interruption of the stele with leaf gaps, at long distances, the cross section above the position of leaf gap will make the stele to appear as horse- shoe-shaped.

Mature (Permanent) tissues 109 C- Vascular system 109 IV- Dictyostele: - This type of stele originates in the same manner as the amphiphloic solenostele type, however, the stem (or rhizome) have leaves at short distances, overlap each other, and this leads to the overlapping of leaf gaps. - A cross section of such stele reveals the presence of separate vascular bundles. These vascular bundles are concentric (central xylem surrounded by phloem, pericycle and endodermis), called meristeles (e.g. Dryopteris). V- Eustele : - This type of stele originates in the same manner as the dictyostele type, however, form ectophloic solenostele, instead of amphiphloic solenostele, to produce collateral

Mature (Permanent) tissues 110 C- Vascular system 110 vascular bundles (e.g. Dicot stems), arranged in ring . VI- Atactostele: - This is a special case of Eustele in which the stele is more disected with leaf gaps, so that , in transverse section, the vascular bundles are scattered though the fundamental tissue (e.g. Monocot stems, Corn & Grasses).

Mature (Permanent) tissues 111 C- Vascular system 111

Mature (Permanent) tissues 112 C- Vascular system 112 Types of Vascular Bundles - The term vascular bundle is applied to the separat groups (strands) of primary vascular tissue. - On the basis of the orientation of xylem and phloem, the individual vascular bundles of the stele are classified into: I- Collateral: - The xylem & the phloem are at the same radius. - The xylem tissue lies closer to the center of the axis, and the phloem towards the periphery of the stem. - Collateral bundles are characteristic of the stems of Dicots. and Gymnosperms but also found in Monocots.