FLYING-TISSUES 



547 



three sub-epidermal layers in addition to the epidermis (Fig. 221 b). 

 Here the cells are oval in cross-section, and have their walls uniformly 

 thickened and lignified ; the cells of the innermost layer are usually 

 much compressed. 



The thin transparent wing of the seed of Spathodea campanula hi 

 is composed throughout of a single layer of flying-cells. These cells 

 are prosenchymatons and arranged in radial rows ; their lateral walls 

 are so greatly thickened as to appear almost circular in cross-section. 



Fig. 223. 



T.S. through the wing of the seed of Zanonia macrocarpa (traversing the thicker 



portion of the wing). 



The actual flying-membrane consists again of the delicate tangential 

 walls, which are stretched over the rigid framework of rods formed 

 by the radial walls. In its natural state this flying-membrane is 

 depressed between the rods to such an extent, that the outer and 

 inner tangential walls meet (Fig. 222 a); the two halves of the 

 membrane can be separated by treatment with caustic potash (Fig. 

 222 b). When examined in surface view, these tangential walls 

 exhibit a delicate oblique striation, which is due to the presence 

 of microscopic fibrillar thickenings. Both the radial and the tangential 

 walls assume a greyish-violet coloration on treatment with chlor-zinc- 

 iodine. 



