EPIDERMAL PAPILLAE. MARGINAL PITS 



121 



Fig. 30. 



leaf. The ray a"b", finally, which impinges vertically upon the leaf- 

 surface is indeed very largely reflected at b" ; the reflected portion is, 

 however, not lost, but is merely diverted into a neighbouring cell. A 

 leaf with a papillate epidermis can thus absorb a greater proportion of 

 the available light than one 

 in which the outer epidermal 

 wall is flat, with the result 

 that both photosynthesis and 

 transpiration are accelerated. 

 According to Stahl, this con- 

 clusion is borne out, so far as 

 transpiration is concerned, by 

 the fact that velvety-leaved 

 plants are almost entirely con- 

 fined to very humid tropical 

 districts. 



The pits, which sometimes 

 occur in the outer wall of the 



Diagram of a papillose epidermal cell, showing how the 

 epidermis, are probably COn- papilla acts as a light-trap (after Stahl). For explanation 

 . c . , . ,. see text. 



cerned with certain subsidiary 



functions of this layer. Where the lateral walls of an epidermal 

 cell are very sinuous, the outer wall is often furnished with marginal 

 pits, which occupy the troughs of the undulations (e.g. Gramineae, 

 Juncaceae, Cypekaceae and various Dicotyledons). Ambronn 69 con- 

 siders that these marginal pits are, with few exceptions, function- 

 less, and that they arise as a mechanical consequence of the 

 corrugated structure of the lateral walls ; he supposes that the 

 corrugation results in the development of various tensile and com- 

 pressive stresses, which in their turn affect the growth in thickness 

 of the outer walls in such a manner that pit-like structures 

 eventuate. It seems more probable, however, that such marginal pits 

 perforin some definite function ; in a subsequent chapter (Chap. XII.) it 

 will be explained how these structures may be of use in connection 

 with the perception of photic stimuli. Sometimes (e.g. Goffea, Cocculus 

 laurifolius, Ginnamomum aromaticum, Camellia japonica) the whole of 

 the outer wall is provided with scattered or even with numerous pits ; 

 here there is a strong presumption that the pits serve to facilitate some 

 process of metabolic interchange between the plant and the surrounding 

 medium. In the case of the. lianes of the genus Salae.ia, the nature of 

 the process can be indicated more precisely. These plants emit un- 

 usually large quantities of water, in the liquid form, from both sides of 

 their leaves at night ; since specialised hydathodes are absent, the 

 water presumably exudes through the pits which are present in large 



