120 



DERMAL SYSTEM 



other instances the interlocking- structures are ridge- or peg-like out- 

 growths of the outer epidermal walls or of the cuticle; this "cuticular" 

 type of suture is well illustrated by Hcclera Helix (Fig. 29 b). In the 



IiHizoPHORACEAE, and iii certain 

 other plants, both modes of union 

 occur side by side. As a rule, 

 perigonial sutures ultimately give 

 way, owing to the pressure exerted 

 by the growing internal organs. 

 Occasionally, however, the lateral 

 union is so strong that the peri- 

 anth is either thrown off as a 

 whole (e.g. the corolla of Vitis) or 

 else remains closed altogether. 



Very frequently the outer wall 

 of each epidermal cell protrudes' 

 in a slightly papillose manner ; in 

 many shade-plants this tendency is 

 accentuated, so that large conical 

 epidermal papillae eventuate. The 

 papillose condition is correlated 

 with certain optical functions which 

 devolve upon the epidermis in these 

 cases. The author 07 long ago sug- 

 hm (after Raciborski). a cuticuiar crested that such epidermal papillae 



re from the perianth of Hedera Helix. o I tr r 



might act as condensing lenses, 

 concentrating the available light upon the chloroplasts which line the 

 lateral walls of the palisade-cells provided the latter were suitably 

 arranged and thereby producing an increase of photosynthetic 

 activity ; at the present time he inclines to the opinion that this 

 " lens-action " of epidermal papillae is primarily connected with the 

 perception of photic stimuli (cf. Chap. XII.). Stahl 68 believes that 

 the conical epidermal papillae of velvety leaves, besides facilitating the 

 removal of water from the leaf-surface, also " enable the leaf to absorb 

 very oblique rays of light, which would be entirely lost if the outer 

 walls of the epidermal cells were of the ordinary flat type." The 

 accompanying diagram (Fig. 30) illustrates the manner in which such 

 epidermal papillae act as light-traps. The oblique ray ab, which would 

 for the most part be reflected from a flat leaf-surface, here meets the 

 sloping wall of a papilla at right angles, and thus penetrates without 

 appreciable deflection through the adjoining epidermal cell into the 

 underlying photosynthetic tissue. The ray a'b' suffers total internal 

 reflection at c, and thereafter passes vertically downwards into the 



Fig. 29. 



A. A cellular suture from the calyx of Boisdu 

 vallia 

 sutur 



