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.
In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word ÏÎ»Î¿Î¹ÏÏ (phloios) meaning bark.his causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect. Some plants, however, appear not to load phloem by active transport.