026 



SENSORY SYSTEM 



Hence, in vertical illumination, there will be a dark patch on the inner 

 wall beneath every pit. In oblicpue light these dark patches become 

 broader on the side of the inner wall facing the source of light, while 

 they simultaneously diminish in width or vanish altogether on the 



Fio. 263. 



A. Vertical section through the adaxial foliar epidermis of Banistcria gplendens, 

 showing marginal pits. B. Vertical section through the adaxial foliar epidermis of 

 Hyperbuena Inurifolia, showing marginal fissures. 



opposite side. The photic stimulus therefore once more consists in a 

 change from symmetrical to asymmetrical illumination of the sensory cells. 

 In a few plants, separate marginal pits are replaced by narrow 

 continuous marginal fissures, which are also directed obliquely outwards 

 (e.g. in Abuta concolor, Hyperbaena laurifolia [Tig. 263, b] and 

 Anomospermum reticulatum) ; in this way continuous reflecting surfaces 

 arise which produce differences of illumination in vertical and oblique 

 light identical with those caused by papillose protrusion of the outer 

 epidermal walls. Similar optical effects result if the radial walls are 

 twisted (Hoy a carnosa, Maranta setosa). 



3. Localised organs of light-perception (ocelli, etc.). 



In the majority of diaheliotropic leaves the entire upper epidermis 

 is developed as a light-perceiving sensory epithelium. Not infrequently, 

 however, the margin, or some other circumscribed portion of the leaf- 

 surface, is specially adapted for the performance of this function 

 (e.g. Tropacohi in ma/us, Campanula persicifolia, Begonia Rex). A still 



