112 



BOTANY 



vAur I 



In many cases, slimy or sticky excretions are produced between 

 the thickening layers of the epidermis and the cuticle ; these press up 



Fig. 110. — Surface view of the epidermis 

 from the upper side of a leaf of M(;rcuri- 

 alis perennis. (x 300.) 



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



on bud-scales. Sticky zones are fre- 

 quently formed on stems, as in the 

 case of Lychnis viscaria and other Sile- 

 neae, as a means of protection to the 

 flowers higher on the stem from unde- 

 sirable visitors. Small creeping 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 fruit trees for a similar pro- 

 tective purpose. Excreting epidermal 

 surfaces form also the nectaries of flowers, which b}' means of their 

 sweet secretions attract such animals, generally insects, as are instru- 

 mental in their pollination. 



The cells of the epidermis are in uninterrupted contact with each 

 other, and as a rule have undulating side walls (Fig. 119). The 

 protoplasm of epidermal cells generally appears to be i-educed to a 

 thin, peripheral layer, and 

 the sap cavities filled with 

 a colourless or coloured sap. 

 Around their nuclei cluster 

 the colourless rudiments of 

 the chromatophores, show- 

 ing that, although exposed 

 to the light, their further 

 development into chloro- 

 ])lasts may cease in cells not 

 destined to take part in 

 the assimilatory processes. 

 Such epidermal cells with 

 undeveloped chromato- 

 phores, 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 

 plants wliich grow in shade chlorophyll is usually present in the 

 epidermal cells. 



The external walls of the epidermal cells of this ujjpcr surface of 

 foliage leaves not uncommonly project or exhibit a lens-shaped thicken- 

 ing in the middle (Fig. 82) ; .such structures are regarded by G. 

 Haberlandt (^^•*) as adaptations serving for the concentration and 



Fio. 120. — Epidermis with stomata from the lower surface 

 of the leaf of Hellehorus niger. ( x 1-20.) 



