546 



MOTOR SYSTEM 



fruits is likewise arranged so as to reduce the specific gravity of the 

 whole structure as much as possible, without impairing its inflexibility 

 and shear-resistance. 



Fig. 221. 



Flying-tissue of the winged seed of Cedrus Libani. A. T.S. through the thin part 

 of the wing. B. T.S. through the thicker region. C. A portion of the thin region in 

 surface view. 



A 



B 



^m^o^i 



The simpler types of " wing " are entirely made up of one or more 

 layers of flying-tissue. Among the Abietlneae the flying-tissue of the 



winged seeds is not, as might be supposed, 

 an outgrowth of the testa, but represents 

 a specialised portion of the ovuliferous 

 scale, consisting mainly of epidermal 

 cells, but to some extent also of sub- 

 epidermal layers. In Cedrus Libani, for 

 example, the thinner portion of the wing 

 is composed of a single cell-layer, which, 

 as already stated, corresponds to a por- 

 tion of the epidermis of the ovuliferous 

 scale ; its component cells are elongated, 

 and have fairly thick radial walls pro- 

 vided with numerous pits (Fig. 221 a). 

 These radial walls form a rigid frame- 

 work with its meshes closed by the thin 

 tangential walls, which collectively con- 

 stitute a continuous flying-membrane. 

 Fragments of the disintegrated hypo- 

 dermal layer adhere to the lower side of 

 the wing in this region. The thicker 

 portions of the wing comprise two or 



Fki. 222. 



Flying-tissue of the winged seed of 

 Spatkodea campan ulata. A. Part of a T.S. 

 through the wing. B. Part of a T.S. 

 through the wing after treatment with 

 caustic potash. G. Small portion of one 

 of the cells of the flying-tissue in surface 

 view. 



