SUBSIDIARY FUNCTIONS OF THE EPIDERMIS 119 



the general physiological and ecological significance of the presence of 

 anthocyanin in vegetative organs is still very obscure. 



Very often considerable quantities of (mm in*" are present in 

 epidermal cells, especially in the case of leaves which persist through 

 the winter. Warming inclines to the opinion that compounds of this 

 nature serve to diminish the risk of desiccation, a danger by which 

 Arctic and alpine plants in particular are often threatened, especially 

 in the absence of snow. Stahl, on the other hand, regards tannin in 

 the first instance as a means of defence against the attacks of snails ; 

 it might also quite conceivably assist in preventing parasitic Fungi 

 from gaining access to the epidermal cells. 



5. Subsidiary functions of the epidermis. 



On account of its superficial position, the epidermis is exceptionally 

 well fitted to undertake a variety of subsidiary functions; this layer 

 accordingly controls, or at any rate plays a prominent part in connec- 

 tion with, many ecological relations which have nothing to do with 

 the protection of the plant. Secondary relations of this kind may 

 modify the character of epidermal cells to such an extent that they 

 lose all or most of the features which are characteristic of protective 

 dermal elements and become transformed into mechanical cells, photo- 

 synthetic elements, and so forth. Any cells which have undergone 

 such a complete change of function can no longer be regarded as 

 epidermal elements in the anatomico-physiological sense, but must be 

 assigned to other tissue-systems. The present discussion is confined to 

 those subsidiary relations of the epidermis which do not effectually 

 mask its tegumentary character. 



The mechanical strength of the thickened outer wall of the epi- 

 dermis is sometimes greater than is required by this layer in its purely 

 dermal capacity. The most obvious illustration is provided by the 

 epidermal cells which occupy the margins of leaves ; these are very 

 often called upon to perform the specifically mechanical function of 

 protecting the edge of the leaf against the shearing or tearing action of 

 the wind, and are accordingly provided with much thicker outer walls 

 than the rest of the epidermal elements. Quite a different mechanical 

 significance attaches, according to Eaciborski, to the epidermal cells 

 which form the sutures between adjacent perianth-leaves in certain 

 flower-buds. 06 There are two ways in which the union between neigh- 

 bouring sepals or petals may be effected in such cases. The epidermal cells 

 may grow out bodily, along the suture, to form a series of interlocking 

 teeth ; such " cellular " sutures are exemplified by the calyces of 

 Onagraceae and by the valvate corollas of Eubiaceae, Asclepiadaceae, 

 Campanulaceae, Loranthaceae, Umbellifekae, etc. (Fig 2 9 a). In 



