72 ON THE STRUCTURE OF THE ROOT. 



cell-walls have become to some extent cuticularised by the impreg- 

 nation of part of 'the cell-wall with cutin, a substance something 

 like cork and impervious to water. In the stem there is a distinct 

 layer outside the cortex, set apart to perform this function ; but in 

 the root the layer which corresponds to the epidermis in the stem 

 is used for another purpose, and the function of protection is 

 taken on by the outer layer of cortical cells. If we look carefully 

 enough, we shall see outside these cells a thin layer of a somewhat 

 ragged brown substance (Fig. 2,pl.), which is all that remains of 

 the outermost layer of the root corresponding to the epidermis of 

 the stem. This outermost layer gives rise to the root-hairs, and is 

 therefore called the piliferous layer, and its function is to absorb 

 water and salts in solution. 



Immediately inside the protective layer are several layers of 

 cells somewhat circular in outline, and so arranged as to leave 

 larger or smaller spaces between the cells (Fig. 2, c). These cells 

 extend up to the limits of the central cylinder, and are called the 

 cortical cells. They contain large quantities of water, and the 

 interspaces may be regarded as air-spaces, through which air can 

 reach the various tissues. The cortex sometimes acts as a store- 

 house for food, and in some roots is the seat of the formation of 

 secondary structures, such as cork. The water contained in the 

 cortex can be taken up when required by the plant, and through 

 the cortical cells water is conducted from the root-hairs to the 

 central cylinder. 



The innermost layer of the cortex is called the endodermis 

 (Fig. 2, e) — that is, inner skin — in contradistinction to the outer 

 skin or epidermis. This layer has a peculiar structure. The cells 

 are similar in shape and size and are closely fitted together like 

 bricks, so that there are no air-spaces at all between them, and the 

 communication between the air-spaces of the cortex and the 

 central cylinder is completely cut off. Furthermore, these cells are 

 cuticularised, but in the majority of cases the radial walls only are 

 changed in this way, and not the tangential, inner and outer walls. 

 Water can thus pass through the cells in a radial direction from 

 the cortex to the central cylinder, but not sideways from one cell 

 of the endodermis to another. In the lime, however, the endo- 

 dermal cells appear to be cuticularised all round except in that 



