During the 7th century the power of the Eastern Roman (Byzantine) Empire was challenged by the Sassanids of Persia, who invaded Egypt in 616. They were expelled again in 628, but soon after, in 642, the country fell to the Arabs, who brought with them a new religion, Islam, and began a new chapter of Egyptian history.
Bark comprises all the tissues outside the vascular cambium of a vascular plant. The majority of the bark of woody plants develops from three meristems: the vascular cambium that gives rise to the secondary phloem, the phellogen that gives rise to the cork and dilatation meristem that produces parenchyma cells to avoid cracking when the axis increases in diameter.
The bark of a tree is a mixture of old, nonfunctional phloem and the young functional phloem (periderm). Xylem: There are two types of conducting cells in xylem, tracheids and vessel elements. Both have thick lignified secondary walls and are dead at maturity.round tissue has a wide variety of functions, even though it is composed of fairly simple tissue types. Parenchyma Tissue: The most abundant, diverse, and versatile cells in a plant are found in the parenchyma tissue.