554 CONSERVATIVE ORGANS. [LECT. X. 



formed by the vascular fasciculi, which it en- 

 closes, have each one half darker than the other, 

 and the darker half is turned towards the upper 

 disk of the leaf. The same circumstance cha- 

 racterizes the fasciculi enclosed in the costse 

 (5. b. b. b. b.) ; and also those of the intermediate 

 or secondary ribs, which form no prominence on 

 either surface of the leaf. By the aid of more 

 powerful glasses, the distinct character and dis- 

 position of both kinds of vessels are perfectly per- 

 ceptible: and in placing the principal fasciculus 

 of the midrib, detached from the others, under the 

 microscope, this is not only satisfactorily demon- 

 strated, but we become acquainted, also, with the 

 fact, that the spiral vessels (6. a. page 553) of the 

 leaf as well as those of the stem, are found ge- 

 nerally empty, like the arteries of animals; while 

 the proper or returning vessels (6. b.) are always 

 full. I may here remark, that the closer proxi- 

 mity of the spiral vessels to the upper disk of the 

 leaf is common to the majority of leaves ; for, in- 

 dependent of the fact, that the chief function of 

 these organs, namely, the exposure of the sap to the 

 light and air, would lead us, a priori, to conclude 

 that the vessels carrying forwards the sap must, 

 necessarily, be on that side of the leaf most exposed 

 to these agents; the sap- vessels receiving their 

 origin in the stem from the vessels of the albur- 

 num, and the returning vessels terminating in 



