82 THE EPIDERMIS [CH. 



The typical epidermis is seen in magnified sections 

 through most ordinary herbaceous stems of seedlings or 

 annuals, or the current year's shoot of perennial plants, as 

 well as those of leaves, sepals, petals, anthers, carpels, &c., 

 as the outermost continuous layer of cells, fitting together 

 without intercellular spaces (excepting at certain definite 

 spots where the stomata occur : organs with which we 

 shall be concerned in a later volume), more or less rect- 

 angular in section, and with the outermost walls thicker 

 than those inside. If stripped or shaved off, and viewed 

 from the outside, the shapes of the cells vary according 

 to the plant or organ examined, giving characteristic 

 appearances ; on the shoots of Dicotyledons, for instance, 

 the cells are usually elongated in the direction of the long 

 axis of the organ, whereas on the leaves the cells are 

 commonly irregular with sinuous boundary walls. In 

 the former case the general shape of each cell is more or 

 less prismatic, in the second tabloid. 



The peculiarities of the outermost walls of the epi- 

 dermal cells give us the clue to the primary functions of 

 the layer, which are, to mark off what belongs to the 

 plant from what is the environment, and to prevent 

 indiscriminate exchanges of gases and vapours between 

 the two. 



This is best illustrated by what occurs in the parts of 

 land-plants exposed to the air with its rapidly varying 

 conditions. 



The outermost cell-walls of the epidermis of a leaf, 

 for instance, not only form a continuous coat over the 

 whole organ, but the external layers of such cell-walls are 

 extremely impervious to water and gases, owing to their 

 being more or less altered in chemical and physical proper- 

 ties by the formation of cutin or cork-substance in them : 

 this altered layer is known as the Cuticle, and the parts 



