sentences:  The earliest macrofossils known to have xylem tracheids are small, mid-Silurian plants of the genus "Cooksonia".However, thickened bands on the walls of isolated tube fragments are apparent from the early Silurian onwards.Plants continued to innovate ways of reducing the resistance to flow within their cells, progressively  increasing the efficiency of their water transport and to increase the resistance of the tracheids to collapse under tension.