 Xylem cells are dead cells, having undergone programmed cell death as they mature after cell division to form a continuous network of hollow xylem. There are two types of xylem cells called tracheids and vessels. Tracheids are smaller in both diameter and length, and they taper at each end where they connect with the next tracheid cell. Vessels between tracheid cells enable passage of water between these narrow cells. Vessels are larger in diameter and length, forming a stacked column of hollow cells much like a straw. Water is transported passively through xylem cells in a continuous column according to the cohesion tension theory, which relies on cohesion of water molecules to one another to pull the unbroken column of water up the xylem. As water is evaporated through transpiration through stomata at the leaf surface, the water is pulled in a continuous column through the xylem, dragging the water column up the length of the plant. This process generates high negative pressure within the xylem cells. Therefore, xylem cells are reinforced with lignin for strength and structure. In drought situations, higher pressure or water tension can cause the water column to break, which incorporates air into the xylem. This is referred to as an embolism or cavitation and restricts further movement of the water through the xylem, worsening the ability of the plant to remain hydrated. The flume is comprised of a column of hollow living cells called sieve tube elements or simply sieve elements, which are accompanied by a companion cell. The sieve elements are connected by perforated walls to form a channel for the movement of sugars and other molecules, such as amino acids and minerals. The flume sap can move both up and down the plant, unlike unidirectional xylem. Think of the flume like a highway. Its primary role is to transport the goods, in this case sugars, from the factories or leaves, to the consumers or grocers, which in this case are the other parts of the plant where they are required. Transport through the flume is complex, but it is generally considered to be an active mechanism driven by an osmotically generated pressure gradient. This process is described as mass flume loading. As products such as sugars build up around the source or the leaves, water molecules are drawn into the flume from the xylem to these regions of higher solute concentrations. As the pressure builds up from the entering water, the flume increases in pressure, allowing the contents to be pushed down the pressure gradient. The sugars and other nutrients are released or unloaded at their sinks, for example the roots or developing flower. The unloading process can be active, such as through the use of sugar transporters or passive via diffusion.