92 



BOTANY 



thickening layers of the epidermis and the cuticle, which press up 

 the latter and finally burst it. Such excreting surfaces often occur 

 inside buds. Sticky zones are frequently formed on stems, as in the 

 case of Lychnis viscaria and other Sileneae, as a means of protection to 

 the buds higher on the stem from undesirable visitors. Small creep- 

 ing insects, which would otherwise rob the flowers of their honey, 

 seem as little able to pass beyond such a sticky zone, as other larger 

 animals to surmount the rings of tar often placed around the trunks 

 of trees for a similar protective purpose. Excreting epidermal surfaces 

 form also the nectaries of flowers, which by means of their sweet 

 secretions lure such animals, generally insects, as are instrumental in 

 their pollination. 



The cells of the epidermis are in uninterrupted contact with each 

 other, and the general firmness of the whole epidermis is also greatly 

 enhanced by their undulating side walls (Fig. 101). In plants with 



Fig. 101. — Surface view of the epidermis 

 from the upper side of a leaf of Mercuri- 

 alis perennis. ( x 300.) 



Fig. 102. — Surface view of the epidermis from 

 the 'Under side of a leaf of Impatiens parvi- 

 flora, showing stomata. ( x 160.) 



special land and water forms, as Ranunculus, the epidermal cells of the 

 land form alone have the undulating side walls. In the delicate 

 epidermal cells of flowers, ridges projecting into the interior of the 

 cells are frequently formed on the inner side of their side walls 

 (Fig. 107). The protoplasm of epidermal cells generally appears to 

 be reduced to a thin, peripheral layer, and the sap cavities filled with 

 colourless sap. Around their nuclei cluster the colourless rudiments 

 of the undeveloped chromatophores, showing that, although ex- 

 posed to the light, their further development may cease in cells not 

 destined to take part in the assimilatory processes. Such epidermal 

 cells with undeveloped chromatophores, besides acting as an external 

 protection, serve as water-reservoirs ; their side walls, by means of 

 folds in the unthickened parts, can expand and collapse as a bellows, 

 according to the variations in their supply of water. In the Ferns 

 and also in several families of the Phanerogams the division of 

 labour between the epidermis and the adjoining tissue is not so 



