576 CONSERVATIVE ORGANS. [LECT. X. 



consists of one fasciculus of spiral vessels, and 

 two fasciculi of proper vessels. The spiral vessels, 

 which form the central fasciculus (d.), are arranged 

 in rays, which are sometimes tangent, at other 

 times separate ; whereas the proper vessels con- 

 stituting the two fasciculi (e. e.), one of which is 

 situated within, and the other without the fasci- 

 culus of spirals, are irregularly embedded in a 

 pulpy parenchyma, and are readily distinguished 

 by their greater transparency. The bark, or true 

 cutis (a.) of the petiole, seems, also, to consist 

 chiefly of several series of the same kind of proper 

 or returning vessels. In the various modifications 

 of this structure of the vascular system, in the 

 petioles of dicotyledonous leaves, the radiated ar- 

 rangement of the spiral vessels is found in all: 

 the petiole in this respect, as well as in the other 

 parts of its structure, closely resembling the 

 stem or the branch from which it springs. In 

 simple leaves, with a few exceptions, although 

 the vascular part forms at first several fasciculi, at 

 the base of the petiole, yet these soon coalesce into 

 one compound fasciculus ; but in compound leaves 

 they remain distinct. 



If we take a leaf of the Elder, Sambucus 

 nigra, and place a transverse section of the com- 

 mon petiole under the microscope, we perceive 

 ten distinct fasciculi of vessels. Five of these 

 (b. b. b. b. b. fig. 11, Plate 8) are compound fas- 



