OCELLI 



027 



more far-reaching division of labour results in the development of 

 optical sense-organs, composed of one or more highly specialised cells ; 

 where these cells differ anatomically from the ordinary epidermal 

 elements, they may be distinguished by the name of ocelli. 



In Dioscorea guingueldba the ordinary epidermal cells have only 

 slightly curved outer and inner walls; interspersed among them are 

 large papillose cells, which may be 

 solitary or collected into small A 



groups. The papillae are conical 

 with rounded tips ; at the apex of 

 each papilla the wall is specially 

 thickened, sometimes to the extent of 

 becoming biconvex. The lens-ex- 

 periment convincingly proves, that 

 the papillose cells condense light 

 much more effectively than the 

 ordinary epidermal elements. 



The best illustration of highly 

 specialised ocelli is furnished by the 

 leaf of Fittonia Verschaffelti (an 

 Acanthaceous plant from Peru). 

 Here the ordinary epidermal ele- 

 ments, which have flat outer walls, 

 are accompanied by a considerable 

 number (about 120-200 persq.mm.) 

 of much larger cells with strongly 

 protruding outer, flat inner, and 

 sloping lateral walls (Fig. 264). 

 Each of these large cells is sur- 

 mounted by a minute biconvex lens- 

 cell, with a very convex outer and 

 more gently curved inner wall. The 

 contents of both cells are perfectly 



transparent, but the refractive index is slightly higher in the case of 

 the small cells. In several respects this two-celled optical apparatus 

 recalls the simple orientating eyes or ocelli of certain animals. The 

 lens-experiment shows very clearly that the structure of the ocelli is 

 such as to produce a very well-marked differential illumination of the 

 inner walls in the larger of the pair of cells of which they are composed, 

 the contrast between the bright central area and the dark marginal 

 region being very marked indeed. 



Ocelli of a very similar type occur also on the leaves of Im/patiens 

 Mariannae. Both here and in Fittonia they must be regarded as 



Fig. 2G4. 



Ocelli from the adaxial surface of the leaf of 

 Fittonia Versclto.fl'elti. A. In vertical section. Z>. 



In surface view. 



