MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



301 



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

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

 the rolling, as for example in the Heaths {Erica caffra, vestita, &c, see fig. 71 1 ). 

 Occasionally a groove is developed which divides into two side furrows running 



Fig 71. — Transverse Sections through Rolled Leaves. 



1 Erica cafra ; X2S0. a Empetrum nigrum; X160. 3 Andromeda tctragona; X150. * Tylanthus ericoides; X130. 



6 Salix reticulata ; x 200. 



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

 71 3 ), and in those of the Cape Rhamnea, Tylanthus ericoides (fig. 71 4 ). 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 broad strong band. On the under side of the 

 leaf, therefore, are three longitudinally elongated parallel projections, a central one 

 under the midrib, and two lateral, which have been formed by the rolled-back margins 



