BIOL 1001 Lecture Notes - Lecture 5: Pith, Phloem, Pericycle

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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: 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. 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. 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: In which phloem surrounds the xylem both externally and internally.

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