102 



BOTANY 



Fid. 109. Surface view of the epidermis 

 from the upper side of a leaf of Mercuri- 

 alis perennis. (x 300.) 



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 fruit trees for a similar protective purpose. Excreting epidermal 

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



sweet secretions attract such animals, 

 generally insects, as are instrumental 

 in their pollination. 



The cells of the epidermis are in 

 uninterrupted contact with each other, 

 and as a rule have undulating side 

 walls (Fig. 109). The protoplasm of 

 epidermal cells generally appears to 

 be reduced to a thin, peripheral layer, 

 and the sap cavities filled with a 

 colourless or coloured sap. Around 

 their nuclei cluster the colourless rudi- 

 ments of the chromatophores, showing 

 that, although exposed to the light, their further development into 

 chloroplasts 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 theunthickened 

 parts, can expand and collapse as a bellows, according to the varia- 

 tions in their supply of water. In plants which grow in shade 

 chlorophyll is usually pre- 

 sent in the epidermal cells. 

 The external walls of 

 the epidermal cells of this 

 upper surface of foliage 

 leaves not uncommonly 

 project or exhibit a lens- 

 shaped thickening in the 

 middle (Fig. 75) ; this ar- 

 rangement serves to con- 

 centrate the rays of light. 

 The refractivity of the 

 lens-shaped thickening is 

 rendered greater by pec- 

 tinisation, ctitinisation, 

 silicification, or waxy secretions. In the cases where the optical 

 apparatus is formed by a bulging of the uniformly thickened wall, the 

 cell-sap containing much tannin serves as the refractive medium ( 1C2 ). 

 The formation of stomata ( 103 ) in the epidermis is characteristic 

 of all parts of the more highly-developed plants which are exposed to 

 the air. Each stoma consists of an intercellular passage perforating 

 the epidermis and bounded by two elliptical epidermal cells, termed 



FIG. 110. Epidermis with stomata from the lower surface 

 of the leaf of Helleborvs niger. (x 120.) 



