MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



801 



If the bent-back margins do not fit so closely together, a groove appears on 

 the under side of tlie leaf, which is more or less sunken according to the extent of 

 the rolling, as for example in tlie Heaths {Erica caffra, vestita, &c., see fig. 71*). 

 Oecasionallj' a groove is developed which divides into two side furrows running 



V.r, 



iflro 



f 



i'ig. 71. — Transverse Sections through KoUed Le.ives. 



1 Erica cafra ; x 2S0. - Empetrum nigrum ; x 160. 3 Androrneda tetragona ; x 150. * Tylanthus ericoides ; x 130. 



5 Salix reticulata ; x 200. 



beneath the rolled edges, as for example in the leaves of Androvieda tetragona (fig. 

 71^), and in those of the Cape Rhainnea, Ti/lanthus ericoirfes (fig. 71*). The central 

 portion of the space framed in by the rolled-back leaves is also frequently divided 

 into two longitudinal grooves, and in such a manner that the tissue below the 

 midrib of the leaf may project as a bi-oad strong band. On the under side of the 

 leaf, thei-efore, are three longitudinally elongated parallel projections, a central one 

 under the midrib, and two latei-al, which have been formeil by the rolled-back margins- 



