MAINTENANCE OF A FREE PASSAGE FOE 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 2 ). 

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



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rm 









wMam 



ra 1 r i 



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H^ 



Fig. 71.— Transverse Sections through Boiled Leaves. 



1 .Erica caffra; X280. 2 Em/petrv.m nigrum ; xl60. B ^i7uironieda tetragona ; X150. * Tylanthus ericoides ; X130. 



fi 5afo'a; retzcWaia ; x 200. 



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

 7l 3 ), and in those of the Cape Khamnea, Tylanthus ericoides (fig. 7l 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 



