560 



CONSERVATIVE ORGANS. 



[LECT. x. 



12. c.), from which the expansion of the leaf 

 spreads out like a shoulder on each side ; we per- 

 ceive that the midrib (12. b.), which is not distin- 

 guishable in the lower part of the petiole (12. a.), 

 becomes very conspicuous on the under disk at this 

 point; forming almost a knob, which passes into 

 a striated ridge, and extends, gradually dimi- 

 nishing in size, to the apex of the leaf. From 

 ten to twelve parallel costae are visible on each 



side of the midrib, 

 22 t> .Jill JL:9L which when magni- 

 fied appear like white 

 parallel lines (13. a. 

 a. a.) , running through 

 the green smooth sub- 

 stance of the expan- 

 sion, and taking the 

 curve of its shoulders 

 as if originating in the 

 white semitransparent 

 space (12. c. c.) al- 

 ready described. But 

 between these costse 

 there are several smaller vascular cords, which are 

 scarcely visible on the surface, neither producing 

 elevation nor difference of colour ; and which can 

 be demonstrated only on the dissection of the leaf. 

 One of the more obvious distinctions, therefore, 

 in the structure of the petiole and the expansion 



2 



