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.
15. The vascular cambium is the secondary meristematic tissue that forms the vascular tissues (xylem and phloem) of the plant in roots and in the stem. Usually, the outer side of the vascular cambium produces a layer of phloem and the inner (more central) side of the tissue produces a layer of xylem.ateral, or secondary, meristems, consist of the cambium and the phellogen (also known as cork cambium). These are tissues in the stem, branches and roots that generate other tissues via mitosis. These tissues participate in the secondary growth of plant, that is, in the thickening of the stem, branches and roots.