MARGINAL PITS 



G25 



Fit;. 262. 



vertical section, or it may correspond to three sides of a polygonal figure 

 (Fig. 262); in the former event the lower half of each epidermal cell 

 is dome-shaped, while in the latter instance it resembles a truncated 

 pyramid. In either case, a central area of the inner wall will be most 

 brightly illuminated [just as happens in the papillose type of epidermal 

 cell] when light falls vertically upon the leaf, while a marginal 

 region (or the side-walls of the truncated 

 pyramid) will receive a smaller amount 

 of light under these conditions. In 

 oblique illumination the distribution of 

 light-intensities is altered, and the helio- 

 tropic equilibrium disturbed. 



This second type of light-perceiving 

 foliar epidermis is exemplified byMonstera 

 deliciosa and many other Aroids, by 

 Hcdera Helix, spp. of Arahis, Fran- 



oiwpa mnrrnmthn e>tr Portion of the adaxial foliar epidermis of 



vv&usu, niUL/uiu/iu, eiu. . Monstera deliciosa in vertical section. 



Very frequently, the characteristic 

 features of the two opposite types of light-perceiving epidermal cell 

 occur in combination, both the outer and the inner walls being more or 

 less protuberant. Cells of this type resemble bi-convex lenses in form. 

 In vertical illumination that is, when the leaf is in the fixed position 

 they are scarcely more efficient than the simpler plano-convex 

 form ; in oblique light, on the other hand, the bright central area 

 shifts on to the inclined marginal region of the inner wall, where, 

 owing to the smaller angle of incidence, the local increase of light 

 is more pronounced than it would be if the inner wall were 

 quite fiat. 



In a number of diaheliotropic leaves, both the outer and the inner 

 walls of the upper epidermal cells are flat. In these cases there are, 

 according to Gaulhofer, 324 various special features which produce differ- 

 ential illumination analogous to that which results from curvature of 

 the outer or inner walls and which consequently enable the epidermis 

 to perceive changes in the direction of the incident light. 



In all such special instances the plant makes use of the optical 

 property of total reflection. A very common arrangement (occur- 

 ring, for instance, in Banisteria splendcns [Fig. 263, a], Aporrhiza 

 panieulata, Anomosperm urn japurcnse, Coccidus laurifolius, etc.] is 

 that in which the outer wall of each epidermal cell is furnished 

 with so-called marginal pits (cf. above, p. 121) ; while varying consider- 

 ably in shape, these pits invariably extend obliquely upwards and 

 outwards from the cell-cavity. Any light that falls from above upon 

 the wall surrounding such a pit, inevitably undergoes total reflection. 



2r 



