sentences:  In early land plants, support was mainly provided by turgor pressure, particularly of the outer layer of cells known as the sterome tracheids, and not by the xylem, which was too small, too weak and in too central a position to provide much structural support.Plants with secondary xylem that  had appeared by mid-Devonian, such as the Trimerophytes and Progymnosperms had much larger vascular cross sections producing strong woody tissue.An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such a structure is Carboniferous.