[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the structure and functions of xylem vessels and xylem fibers in the last video we saw that we find both xylem and phloem in the vascular bundles of plants and if you haven't seen that video then you should watch it now in this video we're looking at the structure and functions of xylem now the first key idea you need to understand is that xylem actually consists of two main types of tissue these are called xylem vessels and xylem fibers we're going to start by looking at xylem vessels xylem vessels start as a series of plant cells running up the stem from the roots to the leaves at a certain point the carbohydrate lignin forms within the cell walls lignin is impermeable and prevents substances from passing through the cell wall the living contents of the cells die and the end walls between the cells break down however regions of the cell wall remain free of lignin these are called pits and these allow water undissolved substances to pass between vessels so the final xylem vessels consist of non-living hollow tubes and the function of xylem vessels is to carry water and dissolve minerals up the plant from the roots to the leaves we'll be looking at how this movement of water takes place in a later video now if a xylem vessel is blocked or damaged then water can move through the pit to different vessels the pits also allow water to move out of the xylem for example the cells in the leaves okay I'm showing you some xylem vessels here with the lignin shown in red lignin can be arranged in spirals or Wings such as the ones shown here in other xylem vessels lignin can be essentially continuous apart from the pits now this lignin helps to support the structure of the xylem vessel when Waters pulled up the xylem vessels this causes the pressure in the vessels to fall slightly and the lignin in the vessel wall helps to prevent the vessels from collapsing now as we said before xylem consists of xylem vessels and xylem fibers xylem fibers are formed from long narrow cells very large amounts of lignin form in these cells and just like in xylem vessels the interior contents of the cells die however unlike in xylem vessels xylem fibers are not used to transport water instead xylem fibers provide mechanical support for the plant so as we've seen both xylem vessels and xylem fibers are non-living tissue however xylem also contains parenchyma cells and we saw these in the last video in the xylem poenchyma cells can act as a store of starch parenchyma cells can also contain tannins which are bitter compounds that deter herbivores from eating the plant in the next video we look at the structure and function of phloem [Music]