Steles in Vascular Plants PDF

Steles The stele of the vascular plants comprises the pericycle (the non-vascular tissues between the phloem and cortex), the primary vascular tissues and the pith, when present. There are two basic types of the steles: A- Protostele Which consists of a solid central cylinder of xylem (i.e. no...

Steles The stele of the vascular plants comprises the pericycle (the non-vascular tissues between the phloem and cortex), the primary vascular tissues and the pith, when present. There are two basic types of the steles: A- Protostele Which consists of a solid central cylinder of xylem (i.e. no pith) surrounded by phloem. Three types of the protostele can be distinguished: 1-Haplostele: In which the xylem appears more or less circular in cross section. 2-Actinostele: In which the xylem is stellate in cross section. 3-Plectostele: In which the xylem is split into longitudinal plates some of which are joined and others are separate. B- Siphonostele In which there is a cylinder of pith within the xylem. Phylogenetically, siphonostele is more advanced and it is thought that it has developed from the protostele by developing a pith inside the xylem. Two types of the siphonostele are distinguished according to the arrangement of the xylem and phloem: 1-Amphiphloic siphonostele In which phloem surrounds the xylem both externally and internally. The amphiphloic siphonostele may consists of a continuous cylinder, of Amphiphloic siphonostele vascular tissues, has considerably distant leaf gabs then termed as solenostele. Leaf gab Leaf trace 16  When the leaf gabs are overlapped, the amphiphloic siphonostele appears as a network of vascular bundles separated with parenchyma cells and termed as dictyostele. Each bundle is an amphicribral vascular bundle and known as a meristele. Dictyostele either monocyclic, dicyclic or polycyclic. In some plants, two or more vascular cylinders are present. Such plants are termed distelic or polystelic plants respectively. 17  2-Ectophloic siphonostele In which phloem only surrounds the xylem externally. Evolutionary, ectophloic siphonostele is considered to be more advanced than the amphiphloic siphonostele. The ectophloic siphonostele may consists of a continuous cylinder of vascular tissues or a network of vascular bundles which separated with parenchyma cells. The bundles are either arranged cyclic (as in dicotyledonous plants) then termed eustele; or scattered (as in monocotyledonous plants) then termed atactostele. Root-stem transition region The primary vascular tissues of the root and the stem differ from each other. The protoxylem in the root is exarch while that of the stem is endarch. The xylem and phloem are arranged alternately in the root, while the arrangement in the stem is usually collateral. At the region where the vascular systems of both the root and stem meet, they become adapted to one another. This region has been termed the root-stem transition region. Four types of adaptation, through that region, have been recognized. 1- Fumaria type Each xylem strand of the root forks radially. The branches twisted laterally and join with the phloem strands. The number of the vascular bundles in the root is the same as in the stem. 18  2- Cucurbia type The strands of the phloem, as well as that of the xylem, fork. The branches of each twisted laterally and meet in pairs. The number of the vascular bundles in the stem is twice that in the root. 3- Lathyrus type Xylem strands do not divide while phloem strands fork. Every two branches of phloem strands join with one xylem strand. The number of vascular bundles in both root and stem is equal. 4- Triticum type Half of the xylem strands fork. The phloem strands unite in pairs. The number of vascular bundles in the root is twice that in the stem. 19