ADAPTATION 225 



size or subdivision of leaves, or in extreme eases by complete loss 

 of leaves and shifting of their functions to the branches; (3) by 

 the development of thick cuticle; and (4) by scaly, hairy and 

 spiny surfac(^s (Fig. 133). The last three serve to prevent rapid 

 transpiration, and so conserve the moisture which the plant con- 

 tains. Desert plants, such as the cacti, are extreme adaptations. 

 They are without leaves and are provided with a moisture con- 

 serving cuticle. Some contain large quantities of water. A desert 

 fern, Notholaena, has wiry stipes and waxy fronds, densely scaly 

 below. More common illustrations are found on the western 

 prairies, where harsh grasses and other plants with harsh, hairy 

 and finely divided leaves are common. The roots of many of these 

 plants penetrate far into the ground, although their branches may 

 extend less than a yard above the surface. 



Flight. Volant animals, since the air is too light a medium to 

 buoy them up, must combine their flight adaptations with others. 

 They may be entirely aquatic and still possess some slight power 

 of flight, but ordinarily they are in some degree terrestrial. Thus a 

 combination of terrestrial, aquatic and volant adaptations in one 

 individual is common, and no animals are exclusively volant. 



Structure. The fundamental requirements for flight are cor- 

 related with the fluid quality and lightness of the air. They in- 

 clude lightness of structure, broad planes for support, steering, 

 maintenance of equilibrium and propulsion, great muscular power 

 and power of endurance. Newman's treatment of "The Bird as an 

 Automatic Aeroplane" is a graphic account of these adaptations, 

 since the birds are the most highly developed flying animals. 



Lightness is secured by the development of hollow bones. 

 Rigidity and strength are maintained, in spite of the relatively 

 fragile bone structure, by their form. Many of the bones are 

 formed like the T and I beams used in structural steel work. The 

 sternum, which bears the great stresses of flight, is an especially 

 fine example of T beam. Fusion and overlapping of bones also 

 add to rigidity of the bird skeleton, while the presence of air at 

 the relatively high body temperature adds to the buoyancy of the 

 entire animal. The feathers form the lightest broad supporting 

 surface known in the organic world, and enclose air about the body; 

 "nearly half of the contour volume of a bird is air-filled" (New- 

 man). 



Organs for supporting, steering, propelling and balancing are 



