LECT. X.] ANATOMY OF LEAVES. 553 



prominent costse. If we now make transverse and 

 longitudinal sections of the Lily leaf, we perceive 

 that the costae are composed of fasciculi of spiral 

 vessels closely accompanied with corresponding 

 fasciculi of proper vessels, and embedded in cel- 

 lular substance; or, that the leaf has a double 

 system of vessels, one for conducting forwards 

 the sap, and the other for returning the proper 

 juice into which the sap has been changed by the 

 functions of this organ. In the transverse sec- 

 tion, these vascular bundles appear like dots upon 

 the divided surface ; and, when magnified in trans- 

 mitted light, display their twofold nature by dif- 

 ference of transparency; the part of each fasci- 

 culus composed of spiral vessels being particularly 

 distinguished by a greater degree of opacity, owing 

 to the spiral thread which composes the coats 

 of these vessels being firmer and more opaque 

 than the coats of the proper vessels. Thus, in 

 the midrib (5. a.), we perceive that the three spots 



