It is generally accepted that the position and development of any one tissue in a plant is to be studied from the point of view of function: nevertheless the physiological need for it may alter as the plant develops. The endodermis is an illustration of this. Its primary function (1) is to restrict the passage of water and soluble substances to certain definite channels: it follows that its best development is in those parts nearest to the source of supply, e.g. in roots and young stems. The cell-walls are at first relatively thin, with the characteristic suberised strip on the radial and transverse walls. Its cells are in uninterrupted contact one with another, there being no intercellular spaces. This leads incidentally to its being also a barrier to gaseous interchange. The ventilating system of the cortex is limited by it internally, and cut off from such intercellular spaces as may lie within.